LiPo batteries, balancers and BMSs

[tvsinesperanto]

First post. Hi All.

Just wondering if anyone can fill in some of the gaps in my knowledge when it comes to LiPo batteries. I've heard good things about their energy density, recharge rates, and open circuit charge retention but I also know that they are a bit fiddly when it comes to management and charging. I'm hoping that someone here will be able to assist me with info about exactly what these batteries require for good maintenance and use.

I am in the process of building my first prototype EV (bicycle based for starters) and, since I expect to go through several iterations before I get something that I'm happy with, I'm trying to keep my costs down as much as possible with this first attempt. Obviously using a cheaper battery chemistry like Pb acid would do that for me quite effectively but there are good reasons why I need to use something with a much higher energy density (I won't bore you with why unless someone is interested). I'm hoping that I can significantly reduce my costs by building my own LiPo charger/BMS (I have a background in Engineering (Electronic and Mechanical) so I'm fairly confident that I can manage it), but first I need to find out exactly what these batteries require, how the commercial chargers work, and what sort of balancing circuitry or management systems are required.

The following is what I *think* I know about LiPos (picked up from what I've read on Wikipedia, various web sites, and just plain old deduction from looking at pics of LiPo packs). I'm hoping that people can take a look at what I have here and add to it where I am correct and correct me where I have it wrong...

1) LiPo cells operate at a maximum voltage of 4.23V and a minimum of 3.0V but are nominally 3.7V. Exceeding 4.235V or going below 3.0V may cause damage to the cell or cause fire/explosion. For this reason, any charger must monitor the voltage on each cell in the pack to ensure that they do not over-volt. Likewise, during discharge (use), the speed controller? (on-board BMS?) needs to have an under-volt cutoff.

2) Like other types of battery packs, LiPo packs have a single pair of high current (thick) wires that are connected across the entire pack (i.e. the red wire is connected to the positive terminal of the last cell and the black wire is attached to the negative terminal of the first cell).

3) However, unlike other types of battery packs, in addition to the high current wires, LiPo packs also have a number of low current (thin) wires equal to the number of cells in the pack minus 1 (eg. if a pack has 6 cells then there will be 5 thin wires in addition to the 2 thick wires mentioned in 2 above. Each of these wires is attached to each individual cell in the pack (Eg. Thin wire #1 and the black thick wire are across cell #1, thin wire #2 and thin wire #1 are across cell 2, etc., etc. right up until the thick red wire and the last thin wire which are across the final cell).

4) The individual taps (i.e. the thin wires) are not connected with a common ground (with the possible exception of those wired in parallel - confirmation required). Each tap wire is at a nominal potential of 3.7V higher than the previous one.

5) When under charge, the charger supplies the bulk of the recharge current via the thick wires and then, after reaching a pre-determined % of full charge, the charger stops supplying this bulk current and trickle charges each cell individually via the thin wire taps.

6) During discharge (i.e. while in use), the current runs to the load via the thick wires only. The thin wires are not connected to anything except when being recharged.

7) A LiPo charger is a fairly complex beast since it has to contain a number of different chargers in one unit. Firstly, it needs to contain a bulk charger that supplies the bulk of the charge (high current at the same voltage as the entire pack - Eg. 24V). Secondly, it needs to contain a whole series of low voltage, low current chargers that trickle charge each individual cell via the thin wires. The charger unit has to contain one of these for every cell in the pack that it is designed to charge. No wonder they are expensive.

That's about as much as I've been able to gather to date. However, even with the stuff I think I know, there are gaps. For example...

1) Do LiPo packs contain any sort of BMS or balancing circuit on the pack itself to ensure that the cells discharge uniformly? If so, what does this circuit look like and how does it work? Alternatively, if it doesn't, how do you ensure that the cells don't drop below 3.0V? Are there special speed controllers for use with LiPo batteries?

2) Assuming I am correct in assuming that LiPo chargers use a 2-stage charge sequence (a bulk charge to the entire pack followed by trickle charge to each cell), what currents are used in each stage? What is the nominal charge current for LiPo cells? Would it be something like 2A during bulk charging and 200mA during trickle? More? Less? I'm having a hard time finding this stuff out.

3) I imagine that most LiPo packs contain cells both in series and parallel. In these cases, do the packs have a tap off each individual cell or do they only have 1 tap at each common point in series (i.e. if it's a 6s2p pack, will there be only be 5 thin wires (1 between each cell in series) or will there be 10 (1 for each cell)?

4) What method do the chargers use to ascertain the charge level of each cell or pack? Do they just monitor the voltage or do they take into consideration other factors?

5) Again, assuming I am correct about the 2-stage charge process, what % of full charge is achieved via bulk charging before switching to trickle charging each cell? How is it measured (voltage level?)?

6) How do the chargers monitor the capacity of the pack as it ages? Again, does it just monitor the voltage or does it use other parameters like internal resistance? Does it even bother monitoring it at all?

I have a lot more that I want to ask but, for now at least, I'll leave it there. Any and all help will be greatly appreciated.

Cheers,
TV

EDIT: I've just read this excellent post http://endless-sphere.com/forums/viewtopic.php?f=14&t=2498 and it answered some of my questions (I did search before posting but didn't associate the term A123 with LiPo batteries - I'd never heard of them before) but there is still a fair bit I'm unclear on.

 

[jondoh]

check out the threads from liveforphysics regarding the progress of lipo vs lifepo:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=8369

Not everyone is convinced that lipos are safe enough for EVs. I'd like to try but i'm building a car next and it seems that lifepo packs for cars are at a lower cost than a home built lipo pack. I'm using 10 milwaukee electric v28 packs on my current electric bike and it's been good to me for over 2000 miles (though i've had to replace several packs under warranty).

to do what you want to to, basically you have to go to hobbycity.com and get 4 or 6 of their 22v, 5ah lipos, connect them to get 66v, 10~15 ah and charge them with 3 smart chargers connected in series. Not cheap but a lot of fun! good luck.

 

[Drunkskunk]

First post. Hi All.


That's about as much as I've been able to gather to date. However, even with the stuff I think I know, there are gaps. For example...

1) Do LiPo packs contain any sort of BMS or balancing circuit on the pack itself to ensure that the cells discharge uniformly? If so, what does this circuit look like and how does it work? Alternatively, if it doesn't, how do you ensure that the cells don't drop below 3.0V? Are there special speed controllers for use with LiPo batteries?

2) Assuming I am correct in assuming that LiPo chargers use a 2-stage charge sequence (a bulk charge to the entire pack followed by trickle charge to each cell), what currents are used in each stage? What is the nominal charge current for LiPo cells? Would it be something like 2A during bulk charging and 200mA during trickle? More? Less? I'm having a hard time finding this stuff out.

3) I imagine that most LiPo packs contain cells both in series and parallel. In these cases, do the packs have a tap off each individual cell or do they only have 1 tap at each common point in series (i.e. if it's a 6s2p pack, will there be only be 5 thin wires (1 between each cell in series) or will there be 10 (1 for each cell)?

4) What method do the chargers use to ascertain the charge level of each cell or pack? Do they just monitor the voltage or do they take into consideration other factors?

5) Again, assuming I am correct about the 2-stage charge process, what % of full charge is achieved via bulk charging before switching to trickle charging each cell? How is it measured (voltage level?)?

6) How do the chargers monitor the capacity of the pack as it ages? Again, does it just monitor the voltage or does it use other parameters like internal resistance? Does it even bother monitoring it at all?

I have a lot more that I want to ask but, for now at least, I'll leave it there. Any and all help will be greatly appreciated.

Cheers,
TV

Welcome to the forum.

1) - Some Lipo do contain a BMS, but not all. Lipo used for Radio controlled planes don't use a BMS. How ever, when recharged, it is done through an external balancer that charges each cell individualy through the taps.
The normal "safe" max charge rate is 1c, or charge current equil to the cell capacity.

2) your're close, the charging is done with a Constant current charge up to peak, then topped off by a constant voltage charge. There is no trickle charge with Lipo.

3) Most packs are series only, but Parrellel is possable. most packs made that way don't contain one set of taps per cell. Thats a shame, as they should to get the best ballance, but it doesn't cause that much of an issue under normal use. If you're building your own pack, its best to build each battery in series only for charging, then hook them up in parrellel if needed for your application.

4)- this depends on the charger and/or the balancer. Some go by voltage only, some use everything from temp probes, to current sensors, some use exoctic algorythems to measure the absorbsion rate of small voltage spikes.


5) - see #2. its not done that way, and it'sy a percentage, but by a measurement. you could have it switch stages from CC to CV at any point, depending on your ability to accuratly measure the cells. the charging rate in the second stage should be almost as high as the first, but limited just enough to not exceed 1c.

6)- the chargers don't moniter the capacity over time, but it is possable to do so manualy. Lipo will recharge at 95 to 98% efficancy, and it's possable to fairly accuratly measure how much current you are putting back into the battery. some chargers have this built in, but a simple meter can be put inline between the charger and battery pack.
You can also just moniter how much power is used with an Amp hour meter attached to the battery. generaly, you can pull greater than 90% of the rated capacity out of a Lipo battery safely when they are new.


Other things to consider.
-Lipo are 40% smaller and lighter than LiFePO4, but have shorter lives, and are explosive.
-Lipo batteries will give you 110% of capacity if you ask them to, but it's fatal for the pack.
-Some people have been able to push the latest generation of packs to 5C while charging with success, but its unclear how this effects the lifespan of the pack yet.
-One of the most common formats for charging is a seperate charger, feeding into a balancer, feeding into the individual cells. But on a high quality battery like those used for RC, it is not nessessary to balance every recharge, and they can be charged through the main power leads.

 

[stew007]

1) - Some Lipo do contain a BMS, but not all. Lipo used for Radio controlled planes don't use a BMS. How ever, when recharged, it is done through an external balancer that charges each cell individualy through the taps.
The normal "safe" max charge rate is 1c, or charge current equil to the cell capacity.

Most chargers now charge through the main leads, the cells are balanced by a small bleed current through the balance taps, and the charging is monitored by the balance taps, so if a cell gets out of line the charger reduces the overall charge current to let the cells balance, or if there is a more serious problem with a cell in the pack, the charger will terminate the charge. This seems the way most of these new chargers do it, and by using this process, it usually ok to charge at 2c provided the pack is 20c+ rated.

as for using the pack, I use a LVC so I have no BMS.

 

[tvsinesperanto]

Thank you all for the very helpful replies. Especially those of you that linked me to more detailed information sources. Very helpful.

From the sounds of it, LiPo might not be the best chemistry to use on my EV after all due to safety concerns, cost, and the finicky nature of the charging process. Perhaps I'd be better off looking into LiFePo4 instead (or even NiMH?)? I'd be interested to know what people would recommend and why they would recommend it in my particular application, which is as follows...

Electric Bicycle (ideally capable of providing 100% of the motive power and not just supplementing your pedalling). I'll probably use a 500-750W front wheel hub motor (although there is an outside chance that I will use a rear wheel motor if I can get a suitable one cheaply).

I'm a tall, somewhat pudgy, bastard (6'4" and 140Kg) and live in a moderately hilly area so I need the motor to be able to deliver high torque (which means the batteries must be able to deliver high currents) for moderate periods of time. The good news is that I would rarely, if ever, go more than a few klicks (say 3-4 each way, at the most) on any one trip so, although I need high peak currents to drag my fat arse up the hills, I don't really need a lot of range. I'll also need to implement regenerative braking which means that battery capacity can be even less than normal.

I am doing everything that I can to keep the costs to a minimum on this because it is likely to only be a prototype. Besides, the wife views all this stuff as "big boy's toys" (her words) and frowns on large outlays of cash on them. When will she learn that these are serious projects? Can I help it if it just happens to also be a hell of a lot of fun?

Anyway, I could save a lot of cash (both on the batteries themselves and on the chargers) by going for lead acid batteries instead of something more advanced but I need to use small, light batteries (and regen. braking) for safety's sake. Using lead acid would add a lot of weight to the bike and that plus my own... ahem... shall we say "ample" frame, will cause issues if I ever have to hit the anchors in an emergency situation (especially if it happens while I'm tooling down a 30 degree grade at 20kph). I doubt that I'd be able to pull it up in anything like a safe distance with all of that extra weight on board if I relied solely on standard caliper brakes. Besides, using all that battery power to get up to top speed and then just using it to heat up the brake pads is a terrible waste.

So, my priorities with batteries are as follows:

- Safety in conditions which may include exposure to the elements and possibly even physical damage to the packs themselves (eg. if there was an accident).
- Low cost
- Light weight
- Simple charging/maintenance routine (i.e I'd prefer not to have to worry about balancing cells after every charge or worrying about cycling them too deeply/shallowly).

...not necessarily in that order.

Is LiFePo4 significantly more forgiving in terms of safety and complexity for recharging/maintaining or does it have the same limitations as LiPo? Also, how does the cost and energy density compare? As I've said, ideally I'd like to be able to do away with all that complex cell balancing and monitoring. Although I'm sure that I could design and build myself a charger for LiPo batteries, I'd really like to avoid it because that sort of complexity will not only be a complete hassle to design and build, but will also be expensive in terms of raw materials (i.e. more components = more cost) and time. Also, the more complex the design is, the more chance I have of screwing the pooch and making an error in the design and/or build which will likely smoke the battery pack and cost me a fair wad of cash to replace. Then there is the fact that I'd have to do a whole heap more research into how the LiPo batteries work, what voltages/currents are required for each cell and how they are applied, what algorithms and other techniques are used for monitoring charge levels and work out how to implement them in my design. Total and utter, 24 carat, hassle of epic proportions and, in the end, very possibly just as expensive, if not more so, than just buying a commercial charger/BMS to start with.

So, will LiFePo4 batteries allow me to avoid all this or am I in the same boat? If so, what about NiMH batteries? I'm pretty sure that the maintenance routine with those is a lot more forgiving (am I right?). Would anyone recommend NiMH (or even NiCd) or am I really better off going for some sort of Li-ion chemistry, despite the complexity? How do they compare in terms of cost, life span, energy density, charge/discharge rates, etc.? Any and all opinions gratefully received.

While we are talking batteries, can anyone put me onto a really cheap source? As I've explained, I'm considering a range of different chemistries so it'd be great if someone could tell me he best/cheapest place from which to source each type. I've heard that sometimes the cheapest way is to buy battery packs for power tools and to cannibalise them. Does anyone have an opinion on this? If this is indeed true, would these packs have balancing circuitry already built in to the packs? If so, would this perhaps be a good, cheap way of getting LiFePo4 batteries and a matching BMS very cheaply?

Are there any other tricks of the trade that anyone is willing to impart? I'm all ears.

In case it matters, I'm in Australia (Sydney) and, although I would consider buying internationally if the shipping costs are reasonable, I'd prefer it if I could get them locally. I'm aware that this may not be possible in all cases though.

I think that that about covers it for now. Once again, thank you to everyone who has replied with helpful info, it's greatly and gratefully appreciated. The combined wealth of knowledge available on this forum is truly awesome.

Thanks in advance,
TV

PS: Oh, by the way, although I'm probably going to abandon the idea of using LiPo packs for this application, if anyone knows where I can get my hands on a circuit diagram, or, failing that, even just a schematic, of a LiPo (or LiFePo4) charger and/or BMS, could you please link me up or otherwise refer me to a source? I'm more than competent enough to build a charging system from scratch (I'm an Electronic Engineer) but I'd need to know exactly what the various battery technologies require by way of voltage, current, monitoring, etc. before I could design anything even remotely workable and, if I had a circuit that I could look at and reverse engineer, that would, in turn, tell me what I need to know about what the battery packs require. I could then build my own from scratch.

PSS: If I did manage to design and build a LiPo or LiFePo4 charger/BMS, I'd be more than happy to put together a "DIY charger/BMS How-to" post so that people could make their own.

 

[Dave-s]

Hi TV,
My advice to you is go with the LiPo. This battery is amazing.
I recently converted my scooter from Lead Acid to LiPo, and am amazed by the improvement.
LiPos have a nearly straight discharge curve whereas SLAs voltage drops as the battery is discharged.
This results in speed and torque reduction.
I too was scared at first that the LiPo will burst into flames, but as long as you dont abuse the battery it is very safe.
I purchased a LiPo charger from HobbyCity that monitors the cells during charging, because I do not have the time to build one from scratch just yet (I too am an EE).
You can build your own charger by using a power supply with a current limit. The top voltage is 4.2v X # of cells in series, and the current is limited to 1C (i.e if the pack is 20Ah, then the max charging current is 20A).
The BMS monitors each cells voltage, and if a cells voltage is higher than 4.2v then it will bleed the cells charge via the tap connector to a resistor.
The BMS also has a a LVC function (low voltage cuttoff) that cuts the current to the load (the controller/motor) when one of the cells drops below 3v.
I do not have a BMS, I rely on the charger to charge the battery correctly, and bought 2 of these: http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=8927&Product_Name=BM-6_Cell_Voltage_Monitor_2-6S_Lipo to monitor my cells (Thank LiveForPhysics).
All in all LiPos are much lighter than SLAs, and you get to go faster, longer (Purket effect) and climb better.
LiPos are about twice as expensive as SLAs, but I beleive that the prices will continue to drop, and in a year or two (next time you will need to replace your batts), the difference will be minor, or LiFePo will a more viable option.

 

[Anonymous]

when you described the performance of the LIPO's...I thought: TERRIFIC.

...my question is about NIMH, ARE THERE ANY possiblities with regard to NIMH packs?

what is the rated c discharge rate for a 48v 20ah NIMH pack?


currently with my SLA's ...I hate with a vengeance.... how quickly the batteries 'SAG' under voltage drain..... the voltage drops down to 35volts AFTER A FRESH CHARGE...(i know..i'm using an etek...300amps is 300amps..).
Lipo seems to be a good option.......... for performance, it's terrific.... bloody tempting...as it's getting cheaper all the time.





1. build an outdoor charging station for the LIPOs.


But.....can you imagine the problems of...let's say...in the event of a crash....Lipos getting crushed by a heavy bike which could precipitate in a large sizzling sound..followed by flames? ......

Hi TV,
My advice to you is go with the LiPo. This battery is amazing.
I recently converted my scooter from Lead Acid to LiPo, and am amazed by the improvement.
LiPos have a nearly straight discharge curve whereas SLAs voltage drops as the battery is discharged.
This results in speed and torque reduction.
I too was scared at first that the LiPo will burst into flames, but as long as you dont abuse the battery it is very safe.
I purchased a LiPo charger from HobbyCity that monitors the cells during charging, because I do not have the time to build one from scratch just yet (I too am an EE).
You can build your own charger by using a power supply with a current limit. The top voltage is 4.2v X # of cells in series, and the current is limited to 1C (i.e if the pack is 20Ah, then the max charging current is 20A).
The BMS monitors each cells voltage, and if a cells voltage is higher than 4.2v then it will bleed the cells charge via the tap connector to a resistor.
The BMS also has a a LVC function (low voltage cuttoff) that cuts the current to the load (the controller/motor) when one of the cells drops below 3v.
I do not have a BMS, I rely on the charger to charge the battery correctly, and bought 2 of these: http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=8927&Product_Name=BM-6_Cell_Voltage_Monitor_2-6S_Lipo to monitor my cells (Thank LiveForPhysics).
All in all LiPos are much lighter than SLAs, and you get to go faster, longer (Purket effect) and climb better.
LiPos are about twice as expensive as SLAs, but I beleive that the prices will continue to drop, and in a year or two (next time you will need to replace your batts), the difference will be minor, or LiFePo will a more viable option.

 

[nomad85]

3) However, unlike other types of battery packs, in addition to the high current wires, LiPo packs also have a number of low current (thin) wires equal to the number of cells in the pack minus 1 (eg. if a pack has 6 cells then there will be 5 thin wires in addition to the 2 thick wires mentioned in 2 above. Each of these wires is attached to each individual cell in the pack (Eg. Thin wire #1 and the black thick wire are across cell #1, thin wire #2 and thin wire #1 are across cell 2, etc., etc. right up until the thick red wire and the last thin wire which are across the final cell).

Actually its number of cells + 1, so a 6 cell pack will have 7 balance wires.

I don't use a lipo charger for my pack 15s2p(3p soon ) I use a power supply modded to 62.4v (4.16v per cell, they last more cycles that way) and a battery medic from hobby city for balancing. I have LVC/HVC boards and a charge controller coming from tppacks.com (Ggoodrum on the forum).

It's not cheap to do lipo safely, but its better than anything else out there. I have been using lifepo4 for 7000+ miles and lipo for about 5, I wont ever go back.