How cheap BMSes shorten your range

[Sunder]

Hi All,

I've been inspired to write this by the number of "My battery is only XXXkm/miles/days old, and I've lost range", type of threads. The answer given by experts is usually "Leave it on the charger overnight", which sometimes works, sometimes doesn't. Let me explain why:

Imagine you have a 14S pack. Most BMSes are designed to shunt excess charge at 4.20 +/- 0.05v, and cuts the charger at 4.25v =/- 0.05v. The problem arises if that +/- bit on those two measurements overlap. Let's say it starts shunting at 4.23v, but cuts charge at 4.22 - both within the tolerances specified.

What happens then is that when the first cell reaches 4.22v, the charger will stop charging the entire pack. In this scenario, you'll have 13 cells at 4.22v, leaving the last cell at 3.94v, or about 70% charged. This cell of course will trigger low voltage protection at the other end, meaning you've lost 30% of your range. In a worst case scenario (where it shunts at 4.25v, and cuts at 4.26 volts), you could potentially lose 90% of your range in a 14S pack. In larger packs, you could completely kill a cell.

The big problem arises because leaving it overnight will not really do anything in these cases - with such a low overcharge, the cells will not self-discharge, and the BMS is not shunting off current, which means the charger will never turn on again.

How do you get around this? There's a few ways:

1. Charge through the discharge port - this port usually is not protected. If you have an adjustable power supply, you might want set the voltage maybe 0.5v higher than the pack voltage, but limit the current to whatever the BMS can safely shunt. (Anywhere from 10ma to 1000ma, depending on quality)

2. Charge or discharge the cells directly, one at a time.

3. Some more advanced BMSes allow you to either change or bypass the cell voltage charge cut, without removing the pack voltage charge cut.

You probably need to be aware that in large capacities with really cheap BMSes, it can take weeks to rebalance - I needed 10 days on the charger 24 x 7 on my motorcycle to rebalance the pack (I used the slight overcharge through the charge port method). Unless you have a really dud pack, you should only need to do this from time to time as you notice your range reducing.

I hope this helps people in the future, if they're wondering why leaving their pack on charge overnight - or even longer, doesn't rebalance the pack.

 

[mighty82]

BMS will cause all kinds of trouble. I always add external jst-hx plugs to the balancing leads so I can check the cells and balance them using a rc charger. With Lifepo4 I remove the BMS and use bottom balancing.

 

[Sunder]

They don't call them battery murdering systems for no reason!

Seriously though, a good BMS can take a load off your mind, even for a risk adverse experienced builder like me. A bad BMS can cause more problems than they're worth.

 

[mighty82]

Too bad there is so few good systems ln the market. For the system to work without overcharging the cells etc. The BMS should be integrated with the charger. A rc charger will never let any cell go past termination voltage.

 

[izeman]

a good working BMS will NOT stop the charging process. it will bleed the high cell, so it won't reach the cut off voltage.
good bms have high power bleeding resistors, even better bms route the power from the high cells to the low cells.
if one cell(string) is a 4.22V and all the others are still at 3.94V then you got a problem. this MUST not happen with a healthy battery that is sized correctly to fit your power needs. under normal conditions all cells should stay more or less perfectly balanced all the time, so a difference like that can not happen.
if your bms stops charge at one cell/string at HVC that's fine and perfectly ok, as long as it starts charging again once the one high cell is discharged low enough to be at a safe voltage. this may take ages for the a big battery to complete, even longer with little bleeding resistors, and even longer with one bad cell/string. but that on the other hand, as said before, shows that you have a bad battery.

 

[Punx0r]

Yep. The BMS gradually brings down all the other cells in the pack while charging the low one, it doesn't just bring the lowest cell up to the level of the rest.

The BMS should be integrated with the charger. A rc charger will never let any cell go past termination voltage.

The problem with this is it means no protection at all on discharge and having to connect balance leads to the charger each time to charge. That's a nuisance and encourages a damaged balance connector, which could then result in ineffective charge protection. A cheap RC charger is also as junky as a cheap BMS.

 

[mighty82]

The BMS draining one or more cell is a big problem for a lot of people that is storing their batteries during the winter season. A friend of mine asked me to take a look at his 20ah lifepo4 pack a couple of weeks ago. The charger would turn on and off while the BMS tried to balance the battery but could never finish the process.

It turned out I had to put 5 amps into one of the 16 cells through the balancing lead before it was fully charged with the rest. The bms can't bring this cells down to the rest, it will try to take 5ah out of all of the other cells which would take "forever". During this process all the other cells will be repeatedly overcharged. Even after charging the low cell it would still balance them all over the place. As long as they ranged from 3.50 to 3.79v it seemed to be happy with the result. Battery murdering system, yes.

A system that could function as a low/high voltage induvidual cell cut off without any balancing function would have been great. One that drains it's power from all the cells equally. I think the need for balancing is mostly there because of the BMS itself.

 

[izeman]

but still what i said is valid: such a high difference between strings only happens with old/bad batteries. they should be inspected anyway. with high quality 18650/a123/rclipo that a sized correctly for the job they just will never be that far out of balance.
i see a non working charger as a safety guard. if it won't charge, then there's something wrong with the battery.
and in the case of a non charging battery after the winter season: a battery needs some maintenance. either you fully charge it at the end of the season (what i don't prefer) or you check it once every 2-3 month and recharge as needed (to maybe 1/2 or 3/4).
but noone tells the regular user how to treat their batteries. everyone understands that a car won't start if not driven for half a year as the battery is dead, but they don't add 1+1 for other similar consumer goods.
if i sell/recommend ebike/parts/batteries the friend/customer needs to get an hour of ear-bleeding explanation what to do and what to avoid. they hate me for that

 

[izeman]

I think the need for balancing is mostly there because of the BMS itself.

sorry. but this is completely wrong. look up discharge curves of different chemistry and you know why.

 

[mighty82]

Should the discharge curves tell me why the cells go out of balance? I don't get it.

I also have the exact same battery pack that he has. Only difference is, mine does not have a bms any more. I removed it and bottom balanced the cells 2 years ago and they are still perfectly balanced at the bottom. This is a 4 year old battery pack. Mine did the same thing with the BMS connected. It drained some of the cells. Lifepo4 cells will not magically drain themself 5ah over a few months without anything connected :?

 

[izeman]

Should the discharge curves tell me why the cells go out of balance? I don't get it.

once you discharge them too low they will go out of balance. weak cells will drain faster and will sooner go out of balance.

Lifepo4 cells will not magically drain themself 5ah over a few months without anything connected :?

no cell will, well nicad maybe . but what's the point? let's say you're talking about 3mo. that's 30x24=~600h. that's 5.000mAh/600h=30mA drain. NO bms that i know of has a drain of 30mA. we are talking uA here, so it would take YEARS to drain that much from a battery.

maybe your bms is broken, or your battery is, and you got a dull cell in it. imho nothing you can blaim the bms for.

 

[mighty82]

If my bms is/was broken I will certainly blame the bms. It was obviously broken from the factory. The same goes for my friends bms which does the exact same thing.

2 years in a row I had to recover 2 of the cells after 5 months in storage. Without the BMS, no problem.

On 18650 packs with various nca chemistry cells I like to use a bms. But I never trust the bms to balance them. I do that myself via balancing plugs and a iCharger once a month or so. The BMS is only there for safety against overdischarge and accidental overcharge. I charge no higher than 4.15v per cell.

 

[Punx0r]

Does sound like a very bad BMS. Those are, indeed, a liability. Any decent BMS should draw negligible power and not from only part of the pack.

Bottom-balancing is uncommon here. It seems to be a bandaid solution to allow discharging without cell level LVC with reduced risk of over-discharging a cell.

 

[Sunder]

a good working BMS will NOT stop the charging process. it will bleed the high cell, so it won't reach the cut off voltage.

This is what I was saying though - A good BMS will do this, and leaving your charger on overnight, or 2-3 days at most, will solve your issue.

The problem I was highlighting is that it seems for a fair few people recently, this has not been the case. I found that out with my own cheap BMS - If the batteries were too far out of balance, the whole pack cut-off was being hit before the high cell drain down was triggering. A really good BMS doesn't even need to hit the 4.20v to balance - it will balance anywhere - it will even take from a high cell to give to the low cell, so you waste less power...

None of these features exist in a cheap BMS, which is often paired with a cheap battery, which gets more unbalanced easily. I'm just suggesting that if this appears to be happening to you, then charging through the discharge port, or manually balancing the cells might bring back your otherwise dead battery.

 

[SniperGaulois]

seeing as charging to 4.2 volts divides the longevity of the battery by 2 compared to charging up to 4.1, i'd say that's more of a range impedence than the bms. How would a cheap BMS would balance 10 cells in series with a 4.1v charger?!? couldnt be too bad?

 

[Sunder]

seeing as charging to 4.2 volts divides the longevity of the battery by 2 compared to charging up to 4.1, i'd say that's more of a range impedence than the bms. How would a cheap BMS would balance 10 cells in series with a 4.1v charger?!? couldnt be too bad?

A cheap BMS couldn't. There is another thread a few days ago with someone looking for a BMS that balances at 4.1v rather than 4.2. The consensus after 2 pages was that unless you were lucky enough to find a LiIon rather than a LiPo BMS, or did a special order, then your only option was a quality programmable one.

Edit: Here is the thread I was referring to: https://endless-sphere.com/forums/viewtopic.php?f=14&t=87501

 

[tomjasz]

How does a fella know a good, from bad, from ok BMS?

A resellr claimed high quality BMS and i bought a pack and spare BMS from China and all three were the same.
Some say Bestech, and then another claims not so good...

 

[circuit]

Check this one out.
https://www.energusps.com/shop/category/battery-management-4

You won't need any external connectors, since all values are available on pc/android.

Sent from my Mi-4c using Tapatalk

 

[Joost]

My personal experiences is too use hand gloves! Not only for your own protection but also for your electronics!
For unexplainable reasons I'm getting charged and when I go out my car or walk upstairs in a shop and touching my car or stairs there is a relatively big spark!!!!

This have make destroying different electronics circuits.
BMS have some small parts that can destroyed by electric sparks.

 

[amberwolf]

That's just static electricity, and they make ESD-control systems for that, starting with the inexpensive wrist-straps, and going up to conductive mats, etc.

(though that doesn't really have to do directly with the topic at hand, which is really about the *design* of the BMS, rather than handling- or service-induced damage).