BMS facts and how to charge lifepo4 without it!

JUICED A single string of chargers.[/quote said:
OK, I guess that answers my question then. Your actually running 16 wires to an 8 cell pack for charging. Does the charges come with any infor on multi cell charging as your doing? What did you say your fully charged voltage is?
Click to expand...
 
OK, I guess that answers my question then. Your actually running 16 wires to an 8 cell pack for charging. Does the charges come with any infor on multi cell charging as your doing? What did you say your fully charged voltage is?
Click to expand...

Yes!!! Much better pic...
I was creating that very early this morning and in a rush.

Much less of a mess going to the box.
I have a 9 pin Molex connector I just plug into each box.

The chargers are isolated so "ganging" them this way works fine.

They charge to about 3.60, then the light goes green. Then it goes into the final trickle (the voltage rises and falls to fully charge the battery) charge untill it reaches 3.65. From what I have read, this is the one of the most effective ways of charging a LifePo4 battery. Much better than a standard single state charger.

The load tests I have done on them with this charger show a better AH capacity.

Juiced
 
JUICED said:
Maybe they will come out with a 10 amp 3.2V LifePo4 charger !!!
Click to expand...

I've built a charger that charges individual cells at 3.65V, 15A per cell. I just used Doctor Bass's idea of using cheap DC-DC converters. The ones I used were bought from eBay for just a few dollars each. They were 3.3V, 15A, ones, that had a nominal 10% trim range. In practice, I found that connecting the trim pin directly to the rail gave an output voltage of pretty near spot on 3.65V, absolutely ideal for LiFePO4 charging.

I hooked these DC-DC converters up to a 36V Meanwell power supply (another cheap eBay purchase). These converters have isolated inputs and outputs, so they can all be connected to the same power source, with their outputs connected directly to a series wired cell stack.

Some have experienced DC-DC converters going into "hiccup" current limiting mode when faced with flat cells, but mine seem to linearly current limit (I'm charging a pack with eight paralleled 10Ah Headway cells, 80Ah in total). Charging this way certainly makes the process a bit quicker, because every cell sub-pack can draw max current up to the point where it reaches the per-cell voltage limit.

The only thing I've found to be a slight problem is lead resistance and the effect it has on maintaining cell balance during charging. It's important to keep each DC-DC converter output isolated, and not be tempted to connect them all in series with just a single wire to each pack cell tap. The reason is to do with circulating currents and the voltage drop in the leads fooling the DC-DC converter regulators. You can have a situation where there's 3.65V at the output of all the DC-DC converters, yet higher voltages across one or two of the cells, due to the voltage drop down the wires (guess how I found this out). To ensure that each cell really only gets the right voltage, you have to ensure that the series connections are only at the cell end of the leads, with two wires from each cell tap point, one leading to the positive of one DC-DC converter and the other leading to the negative output of its neighbour. This reduces the possibility of getting big errors from voltage drops in the cell tap leads, albeit at the expense of a bit more wire.

Jeremy
 
dumbass said:
Andy,

An absolutely great explanation on how it works. Thanks, I really learned a lot from it. Your comments about what I will call pulsed motor response is the same comment that was stated by Jim Olson from Cyclone USA the first time he tried a lifepo4 pack with a 500w Cyclone kit. Ether the pulsed protection or the speed limiting protection sounds like something I would consider. Now that you have done such a great job explaining the discharge protection can you please explain the charging protection? From what I have read (and this is something I don't like) is that the BMS is just bleeding off the voltage on high cells. I have also read that with some BMS they cut off the charge once the first cell reaches a full charge.

You mention “Gary Goodrum's LVC boards”. Is this only a LVC or a BMS with LVC with ether pulses or a reduction is current draw. I like the idea of using individual cell chargers but would like the addition of a low voltage control.
Click to expand...

First of all, Andy has it completely right, with regards to how the LVC works. The only minor correction is that the voltage detector hysteresis is .15V, so a 2.1V detector will reset at 2.25V.

I do have LVC-only boards, which I have been selling, but these are being updated, along with the new v4.0 version of the BMS unit itself, which we are just about finished testing.

A BMS really only has two required functions, cell level low voltage protection, and charge management. If you use individual cell chargers, all you need is the low voltage protection part. I used this exact setup for quite sometime with my a123 and PSI-based packs. I have 16 of the VoltPhreaks 2A individual chargers that I used to "top-off" each cell, or block of paralleled cells, and used a 48V/15A Zivan NG1 bulk charger in parallel with its voltage set a bit below the CC/CV set point of the VP chargers. This works quite well, but the problem is the setup is bulky, and all the connections can be a pain. I fixed the latter issue by using a single 18-pin AMP 4.2mm PE Series set of connectors, the female version wired into the packs, and the male wired to all the chargers. Still, having to drag this whole setup around with my folding bike go to be too much, which is really what got me to hook up with Richard, and start down the BMS path.

From the beginning, the goal has always been to use a single bulk charger and have the BMS work like a bunch of individual cell chargers. This is different than the way RC-type balancers work, which is to simply bleed off current from all the cells that are higher than the lowest cell. Anyway, the behavior of the individual cell chargers that the BMS tries to mimic is the ability to let each cell receive a full charge, at its own pace. The reason this is important is that even cells that are carefully matched when the pack was built will eventually drift apart, and end up with slightly different capacities, internal resistances, thermal characteristics and states of charge. With individual cell chargers, each with their own CC/CV charge profiles, the cells will simply take however long they need to get full. With a bulk charger, charging all the cells in series, this is a problem because when the first cell gets full, it won't let any more current in. Since all the current has to go through all the cells, however, that means the cells that aren't full yet stop receiving current, which causes them to get even farther out-of-balance. SLA cells are different. They have the unique ability to "absorb" a bit more current, even though they are full. The trickle mode of an SLA charger is there to keep the full cells topped off, while allowing some current to flow to let the lower cells catch up. All Lithium-based chemistries don't have this "self-absorption" capability, so when the first cell is full, that's it, no more current flows to the rest of the cells. By adding a shunt circuit to each cell, a certain amount of current can bypass the full cell and flow through the shunt, so that it is available for the next cell in series.

In addition to providing this shunting capability for each cell, the BMS needs to do one more thing in order to mimic the individual cell charger behavior, and that is basically to have a separate CV mode for each cell. This ensures that no one cell (or block of paralleled cells...) goes over the CV set point. Without this feature, the first cell to hit the cutoff will suddenly have its voltage rise and keep rising until the bulk charger's CV mode kicks in. LiFePO4-type cells are more tolerant of this sort of over-voltage condition, but repeated instances of this will eventually affect cell life. LiCo-based LiPo cells, on the other hand, can't tolerate significant over-voltage conditions at all, and can quickly go into thermal runaway, which causes the cell to explode in a fireball and burn at over 2000F until all the Lithium is gone.

Not all BMS designs work the same, when it comes to handling what happens as the cells hit the CC/CV crossover point. What our BMS does is detect when each cell's voltage reaches this set point, and then it trips the same opto-controlled output that is used by the LVC circuit, which is obviously inactive during the charge process. The charge control logic then uses this signal control the duty cycle of a PWM circuit that reduces the current by however much it needs to in order to keep the cell's voltage at the limit. Without the shunt circuits, what would happen is that the first cell to hit the set point would control the current available for all the cells, so that in effect, only the first cell will have a proper CV mode, and will get completely full. In combination with the shunt circuits, however, the net effect is that you have individual CV modes for all the cells, so just like the individual cell charger setup, each cell can get completely full, at its own pace.

-- Gary
 
Since I have these as 8 battery units, any reason this would not work to monitor as I ride?
Has anyone used these yet?
Does it have a audible or visual warning device suitable when riding a motorcycle?

It does 8 cells packs.

View attachment CL8S.jpg

Juiced
 
I have 1/4 scale RC cars running these large LifePo4 Batteries as well as the motorcycles. This unit would be an easier check
by just plugging it in and doing a quick check between Rounds.


Time to get on the waiting list....


Juiced
 
JUICED said:
Since I have these as 8 battery units, any reason this would not work to monitor as I ride?
Has anyone used these yet?
Does it have a audible or visual warning device suitable when riding a motorcycle?

It does 8 cells packs.

Juiced
Click to expand...

The only problem with the visual alarm is it's only good if your looking at it. The audio alarm is not very load and is not latching so it only sounds for about a second. Somewhere on this site there is a demo video of both V & A alarms activated. However, there is an output from the audio alarm that can be connected to a relay that activates a loader audio alarm. Nice but a lot of work. I have a hybrid car and tend to watch the battery and electric motor monitor a lot. So like this meter's visual display so I can keep an eye on what's going on.
 
Thanks for the feedback!


Juiced
 
Here is the set up before I added the Molex connector joining all of them.

Nice and easy to keep an eye on it.

I have since marked them so I know which Pack is "acting up"

View attachment New chargers 0071a.JPG

Juiced
 
I have a 9 pin Molex connector. It make it easy to plug in the 8 chargers into anything I have wired correctly.

http://en.wikipedia.org/wiki/Molex_connector


Juiced
 
Ed, I just checked out your page...you build some cool stuff guy! My garage and basement are full of projects...hovercraft, rc helis/planes/cars, e-bikes, quads, moto-xers. I'm also looking at using solar panels this year for the bike. Probably go with 400watts of panels and deep cycle batterys but also looking at the small grid tie inverter which would likely be more eff. Side note...20" plus of snow today in NJ!
 
Thanks and OUCH Too much Snow for me.
I will post more photos of the battery boxes as get the wiring all cleaned up.

Imagine using Headway batteries in a 1/4 scale RC car..... Got 3 I am finishing up.....

Too many projects!!

Ed
 
Interesting to see I’m not alone in struggling with BMS and charging issues.

I have tried two BMS’s on my Headway 48v 10ah pack that I built. The low voltage cut off worked on both of the BMS’s, but neither would allow for a full charge of the pack. Both BMS also did a reasonable job of protecting the batteries form overcharging. None of the cell voltage exceeded 3.9 volts during charging. The problem is when charging through the BMS a number of the cells never charge above 3.4 volts. I tried charging without the BMS but ran into overcharging issues.

My short term solution has been to use the BMS for low voltage discharge protection and high voltage charge protection. In addition, I periodically balance the pack with a single cell charger.

I recently built a Goodrum/Fechter BMS which I am going to use in rebuilding my 48v Headway 10Ah pack into a 20Ah pack. Hopefully this will result in a worry free, fully charged, balanced, protected pack.

Glad I read this thread. I like the idea of monitoring the voltage of each cell. I’m going to buy a couple cell log voltage monitors. This will make it a easier to test and monitor cells while they are installed in the bike. Unfortunately my installation design doesnt allow for easy removal of the pack. A problem I will address in my next modification.

Question: I have tested the voltage and amperage of my charger to make sure it was working up to spec. I noticed that it has three potentiometers built into the board. One POT controls the voltage and one controls amperage. Does anyone know what the third POT is for? The charger is a LiFePo4, 48 v, 4A made in china charger.

Barry
 
"Question: I have tested the voltage and amperage of my charger to make sure it was working up to spec. I noticed that it has three potentiometers built into the board. One POT controls the voltage and one controls amperage. Does anyone know what the third POT is for? The charger is a LiFePo4, 48 v, 4A made in china charger."

I have couple of the 72 volt versions. Not sure what the other does either and trying to find a schematic is near impossible.

I have watched the charging profile of these chargers and mine will charge to about 3.60 (just like the small ones) per cell and then ramp up and down until all the cells are at maximum. You may want to leave it on longer?

I have not had any issues with the chargers, just the BMS units.

The new and improved (Headway provided) units seem to have gotten better, but due to how I have the bike wired now I will keep the setup I have.


Juiced
 
Here is the battery Pack with the Headway installed BMS.

As you can see, not the best setup for a vibrating motorcycle.

We will be fixing That!!!

In the box is the older version BMS. Good for charging, but not enough capacity for the LVC.
I have received new and improved 72 volt - 24 cell units that do work well. I will be selling all of the new extra units soon.

Notice the JDS installed FUSE!!!

View attachment New chargers 003.JPG

96 batteries.....no waiting!!!! :) :)

96 Cells, all load tested and charged with the single cell units and ALL sitting at 3.33 volts ready for assembly.
They will be set up as 8S4P boxes.
View attachment lifepo4 conversion 003.JPG

Here is the first batch of TOP plates.

**** NOTE!!!****

These cells are all slightly different size in length. Solid top plates are fine if you use the single cell to cell
jumper taps (see first photo) to keep undo stress off of the cells.

View attachment lifepo4 conversion 004a.JPG


More Later!!!!!


Juiced
 
Nice looking industrial strength connectors and bolts.

What gage aluminum did you use? Did you build the battery cases or did they come with the batteries?

Barry
 
Hello Barry,

The aluminum is 1/8 inch plate cut to size and boxes were original from Headway.


Juiced
 
pgt400 said:
pic maybe better....
Click to expand...


am i suppose to look at this from left to right ? i was looking from right to left every time i approached it to try to understand it :lol: but couldnt figure it out, and today after few tries in few days my head said holy sh*t i can look from the other side, it makes more sense, looks like parallel charging ? (just making sure, i must make sure cos if it took me few days to see it from another angle maybe im still missing something). i forgot to notice or more correctly my head didnt brought this fact up from my memory to my attention that i should look at positive negative leads and follow a revers flow of electrons HA HA.


GGoodrum said:
All Lithium-based chemistries don't have this "self-absorption" capability, so when the first cell is full, that's it, no more current flows to the rest of the cells. By adding a shunt circuit to each cell, a certain amount of current can bypass the full cell and flow through the shunt, so that it is available for the next cell in series.
Click to expand...

technical question just trying to understand the work of it, when the cell is full it got 3.65v back EMF against the charger, does the current diverts because the cells BEMF repelles it so chargers current starts flowing through the shunt which is with less resistance, and the cell in this case is like an electrical type of shunt ?

and chargers usually hit bats with more volts, so if total voltage of drained 10s LiFePO pack is 25v and the charger is 40v how is the charger doesnt kill cell by cell, charging first cell full and continuing to overcharge it cos it got more volts to overcome cells 3.65v resistance, is there some component that divides the voltage from the charger ?

if there is something that divides the voltage for the cells anyway, wouldnt it also work with simply having a 4v charger with high amps and BMS that parallely charges the cells ? (im thinking laymen-y here, volts stay same in parallel, amps divide)

or it just complicates the circuitry more this way ?

lets see what other stupid questions i have :mrgreen: , general 1, those balancing boards mentioned earlier, when they are connected to 1 of the charges as icharger, GT A8, turnigy, the balancing bleeds the higher SOC cells ok, but does the charger kicks in again and again after every bleed to top the high cell till the lower catches up, or it simply bleeds the high cell lets say from 3.6v to 3.3v because the lower cell is at 3.3v and all operations stop ?

i saw stand alone balancing boards i think, do they just bleed the top cells to match lower cells so a user could start a charge with level SOC cells ?

are there stand alone balancing boards that dont just bleed but actualy transfer current from cell to cell for balancing ? (i think i saw something like that because it was said the board balances at 300ma and takes 80ma for operation)


say thanks to the guy who linked me to this thread, if i wouldnt read this i wouldnt have the new stupid questions :lol:
 
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