Im not going to bother with a BMS

I'm not going to bother with a BMS, what do you think?

here are my main reasons

* a good BMS is near 60% of the cost of my battery pack
* BMS failures and complications could leave me stranded
* balancing currents are often so small that it takes a very long time to bring the cells in line.

how will i protect my setup ?

* 3 x 8 cell ebay cell monitors to alert me to any issues
* periodic balance charge with a UN-A9 plus

no roadside breakdowns
no complexity
no expense.

what id be interested in is how often i need to balance charge with a new set of 18650 batteries that are all balanced to start with ?

I've been riding lipos for one year and I have NEVER balanced them. The day I "over dicharged" below 3.5v per cell, after a non balance charging, the cells were completely aligned between 4.18v and 4.20v. That is using HK Imax B6, on the other hand if I use HK GT A-6-10 200W the voltages are 4.16 to 4.20v, more imprecise, Why a charger makes a difference? I can't guess why.


Before Lipos I had a recycled laptops 18650 cells, 14s9p, all sort of brands and different C-rate & mah mixed to get the same mah per serie.
No Bms, and over stressing them with a 60A controller. When discharged cells were with up to 0.20v of difference; After charging them they were between 4.03 and 4.08v. I couldn't care less, they did their job and that was the main point.

Probably when you buy new cells, the BMS is helpful to prevent any row of parallel cells with a faulty unit compromising the rest. But the cell monitor you want to buy I guess would do the same job :wink:

As long as your system provides cell-level protection against over-charge and over-discharge then you have the basic safety requirements met. Automatic balancing is a nice-to-have and if you don't need it, you don't need it.

I went for a BMS over cell-monitors because I couldn't be bothered constantly connecting and disconnecting the things (leaving them connected will drain and probably unbalance the pack) and because I don't want to sit there watching the monitors as the battery charges.

Yeh I was thinking the cell monitors would drain the batts, i guess I can set it so they are only active during riding and charging I should be right.

Most controllers now are really our BMS , in terms of LVC , OVC and max draw, cell monswill tell me if a cell row is out.

I often wonder about those cell bleeders u see for the large prismatic cells ,money could be easy set and forget for balancing.nbut they are all lifep04

Actually,, your title should be " I'm going to be a human BMS"

Or you could just ignore the battery, and kill it. You kind of have to have a bms, but it need not be a machine. Hard to say which is more "bother".

How often will you need to balance? Every time you screw up, and they get unbalanced. Every charge, if the cells are garbage.

Here's the key points to less hassle when you are the bms.

Get rid of shitty cells.

If any cells have low capacity, high internal resistance, or self discharge that will not stop, chuck those shitty cells. DONT try to live with them.

Slightly undercharge.

If your target is 4.2v per cell, and the ruined cell voltage is 4.3v,, charge and balance to no more than 4.15v. This buys you a bit more wiggle room at the top. Now you don't have to shit a brick because your cells are .02 out of balance, you have .05v to play with, or even .15 before disaster. It will cost you capacity, but not as much as charging to 4.1v will.

Easy on the discharge.

Discharge at low c rate. For cheap cells, .5c. For hobby lipo, 2 or 3 c. This will help the battery keep its cool. If the pack gets hot, not just warm, lower your c rate. In addition, stop early. Where ever the cliff is for your cells, stop before it gets too close to that abrupt voltage drop. Do those two, and you can ride for months with no need to manually balance the cells.

Now you can just bulk charge, check from time to time with a cellog 8, and run nothing more than a watt meter or voltmeter on the bike. No need for alarms if you know what pack voltage to stop at, and the pack stays balanced.

When I do need to balance my naked, hobby king packs, I do it one cell at a time. I use a cheap RC charger to charge or discharge at 2 amps or less, through the balance plug. No need to discharge 4 cells 15 times taking hours, charge the low one. done.

I have 880 charge cycles no bms. I have replaced two sets of sense wires and need to replace a third set. Do I check my battery yes it's religious. Come to think about it, it owns me. 4yrs. I also have A123 heavy high quality 20ah cells.one cell ez to monitor not 5p. Just 1p.but just use 17ah at most.

Hey I'm planning to do the same too! I just bought a RadioLink CB86 Plus 6A 210W Lipo Balance Charger that has 8x 6S charging ports so i can just leave them all connected and it will charge the cells sequentially, but it is still in the mail and I'm planning to use it with in a either a 20s1p config with 3.2V 20Ah LiFePO4 cells or a 16s1p config with 3.7v 20Ah Li-ion cells that are rectangular in shape and can be bought from taobao.com (China's ebay, beware of fakes and used batteries posing as new)

Jestronix said:

I'm not going to bother with a BMS, what do you think?

here are my main reasons

* a good BMS is near 60% of the cost of my battery pack
* BMS failures and complications could leave me stranded
* balancing currents are often so small that it takes a very long time to bring the cells in line.

Click to expand...

If your battery is that cheap, I'd say the BMS is not the weak point. I don't have a low voltage warning on my scooter battery, but I do have balance bleeders which effectively manage high voltage cells. They trim each LiFePO4 cell down from wherever it is to 3.55 V each. I never go much further than 40 km in a day and the range is good for almost 60 km. But I do have a voltage display on the dash, so that's me playing the role of LVC.

Balancing/bleeding is not necessary, but you do need to keep an eye on them periodically. A celllog8 with a convenient connector will probably work for the most part. Cheap too.

Otherwise, buy a BMS. They aren't that much trouble.

Theres BMS and there's good BMS from what I can see, and as u go up in amps things go up. For instance I would need a 24s with 100 amp rating, my battery is 800whrs , sourced from on discount tool batteries it set me back $400 usd. Price for a reasonably good BMS ? I'm guessing atleast $200 bracket ? That's a 50% hit.

I’ll throw out the concept that some high voltage lawn tool manufacturers have adopted with their commercial battery packs.

Cell level BMS protection for charge only. Discharge is not as potentially damaging/menacing for equipment/tool/eibkes used outdoors. If cells are kept reasonably balanced the controller LVC can provide sensible discharge protection. In some cases a simple pilot lamp can also be plugged into the charge port to alert operator to a cell level problem.

When you think about things, it’s much more serious and menacing to persons/surroundings charging packs indoors. Often, while unattended.

In the scheme of things, BMS become very expensive to handle high discharge current. On the other hand, charge current will commonly be under 5A and that sort of component count is much cheaper to produce a BMS for use as charge only protection circuit. In other words, you can use a much smaller BMS to cell level protect on a dedicated charge port. Discharge directly off the battery terminals.

I’ll spam a link to my recent thread about this concept and some dirt cheap 4S BMS boards which are behaving very nicely when used in series for my 16S packs.

https://endless-sphere.com/forums/viewtopic.php?f=14&t=82262

Aloha all. I have 4 x 4s 5.00 ah hard case lipo cells (16s in all). I have voltage LED displays on each of the 4 packs and watch to make sure if one pack drops below the others or is not even.

I also use this to watch voltage sag, and adjust my throttle (lead foot) according to the sag. I try to keep the 4 pack cells above 14.50V all the time, so when I am nearing the end of my mileage, I may get only 1/4 throttle @ 14.50 v whereas at full charge I can WOT to 14.5v. I believe the sag level is the most important.
So to me an automatic BMS is not needed at all and use a balance board when rarely needed and stay under 4.15 full charge and over 3.6V.


Francis

True enough,, start with good cells, weed out bad ones, then you have very little need for discharging through a bms. Let an LVC do it, or monitor pack voltage and stop when you choose to.

But a bms can be connected permanently for a balancing charge most cycles. And you can have two chargers, or one with the three level switch, so you can practice an undercharge most cycles. Just bypass it to discharge, then you don't need 100 amps rating.

The main criteria would be a decent quality bms or balancing board, with HVC, and not running by draining just one cell.

As always, trust but verify. Catch a defective bms before a broken balancing circuit drains a cell group to it's death. If you are doing the most cycles undercharge thing, plug it in only when you need a balance, about 4 times a year. But if you want safer charging, then leave it connected all the time or at least every charge,, so it cuts off the charge when cells overcharge, if ever.

I wonder why the most charger do higher than battery ratings.

Why is this? My charger outputs ~42.v to my 36v Li-Ion-pack.

If it have no BMS the charging can go on for days to the cells because the charger never goes green.

James_B said:

I wonder why the most charger do higher than battery ratings.

Why is this? My charger outputs ~42.v to my 36v Li-Ion-pack.

If it have no BMS the charging can go on for days to the cells because the charger never goes green.

Click to expand...

Battery packs (cells) operate across a wide voltage range.

Most recent 18650 packs use cells which range between 2.8V-4.2V. 10S (series) 36V pack delivers operating voltage range of 28V-42V.

To fully charge the pack in my example, the charger will need to reach a maximum output voltage of at least 42V. In many cases that will be even higher if the BMS is supposed to balance the pack.

I'm not sure I understand your last statement regarding BMS but if you're charging without BMS make sure they're balanced and maintain a close watch on individual cell voltages.

I only dont use a bms is because of 20ah I wouldn't use less then 15ah of high quality cells. So high quality cells 15ah or more, sense wires for daily checking. I use a cell balancer if needed. Maybe 4 times a year. Plus a cycle analyst. That's a minimum. Oh I bulk charge each 12s @ 16 amps with two charging setup's. Did I say high quality cells.

I don't use a BMS because I always balance charge with my i3010B which is fast as shit.

And my controller AND cycle analyst can limit battery current, so the BMS isn't useful for that either.

James B might have a mis match between charger and battery, or it's a broken charger.

999zip999 said:

I only dont use a bms is because of 20ah I wouldn't use less then 15ah of high quality cells. So high quality cells 15ah or more, sense wires for daily checking. I use a cell balancer if needed. Maybe 4 times a year. Plus a cycle analyst. That's a minimum. Oh I bulk charge each 12s @ 16 amps with two charging setup's. Did I say high quality cells.

Click to expand...

What are these high quality cells, a123 ?

Jestronix said:

Y

Most controllers now are really our BMS , in terms of LVC , OVC

Click to expand...

Controllers provide no protection against over-charge of the battery. As pointed out above, using a BMS for charge-only protection is sensible, easy and cheap and addresses the main safety concern.

i ALWAYS use a BMS. it's a no brainer. plug&play. check individual cell's voltage every few month and all fine. i don't think that $70 is too much money for a comfortable charging experience and some battery safeguard. the battery was $700 and the bike was $3000 with 100s of hours work. so it's only a few percent of the whole build and weight it negligable.

Yes A123 20ah. I had so many problems with singalab bms's and saw a lot problems on pedago's. I was using two 1220 and always balance charge for two yrs till they stopped working and a hyperion 1420 also. Now a meanwell and hp power supply bulk charging and daily checking

izeman said:

i ALWAYS use a BMS. it's a no brainer. plug&play. check individual cell's voltage every few month and all fine. i don't think that $70 is too much money for a comfortable charging experience and some battery safeguard. the battery was $700 and the bike was $3000 with 100s of hours work. so it's only a few percent of the whole build and weight it negligable.

Click to expand...

That certainly is a good price, what BMS can u recommend for my 33s pack ?

999zip999 said:

Yes A123 20ah. I had so many problems with singalab bms's and saw a lot problems on pedago's. I was using two 1220 and always balance charge for two yrs till they stopped working and a hyperion 1420 also. Now a meanwell and hp power supply bulk charging and daily checking

Click to expand...

A good example ! I'm proabably only going to charge to 4.1 or 4.0 leaving some safety room at top, I charge outside, my bike lives outside, I always charge while I'm at home so I'll hear the buzzers go off if there's a problem

I'd never charge inside ! Even with a factory solution, it's a gamble.

dogman dan said:

Here's the key points to less hassle when you are the bms.

Get rid of shitty cells.

If any cells have low capacity, high internal resistance, or self discharge that will not stop, chuck those shitty cells. DONT try to live with them.

Slightly undercharge.

Click to expand...

+1

Maybe I should still clarify,, In some cases guys build a pack with all shitty cells. None self discharge and won't stop, but all have high resistance and low capacity. In that case, they are all the same, and you can still maintain balance with careful discharges. Not too much c rate, not getting too close to 100% DOD. and undercharge enough to have some headroom at the top, since you have no overcharge protection if using a dumb charger.