can I change a cheapo BMS to be 4.1 volts / cell?

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marka-ee   10 mW

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can I change a cheapo BMS to be 4.1 volts / cell?

Post by marka-ee » Jul 28 2020 4:13pm

I have a couple of the el-cheapo-ebay-china BMS's that are design goal wise set to 4.2 for the balance voltages. I like to run my packs at a max of 4.1 V/cell to promote pack health. So maybe I can change it to 4.1 ? Maybe a bunch of resistor changes or something ? I'm just shooting in the dark. I do have one of the programmable 'ANT' type BMS also, but I'd like to get some use out of the more compact and cheaper 'dumb' BMS's as well. Side question : Why do they all seem to use 4.2 as max voltage ? Great way to shorten a packs life. Imagine if EV cars did something so dumb, always charge to the max cell rating.

Balmorhea   10 kW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by Balmorhea » Jul 28 2020 5:10pm

Yeah, who cares about having maximum range? We're just like stinking car drivers; we can carry as many thousands of pounds of batteries as we like.

If you want control over the BMS parameters, get a programmable BMS. Otherwise, expect to exercise your own discretion about how high to charge and how low to discharge.

goatman   1 MW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by goatman » Jul 28 2020 6:29pm

why not use the cheap bms and only balance charge to 4.2v when you think you need to, the rest of the time just charge to 4.1v

i dont use a bms and check my batteries about every 2months, which reminds me, ive got 2500kms on my trike and should check the batteries, but they seem fine according to my CA :lol:

john61ct   100 GW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by john61ct » Jul 28 2020 7:40pm

Get a BMS that does what you want.

There are hundreds of ones that won't.

Cheap BMS should have nothing to do with stop-charge control

If you don't have a real charger, or its setpoint is not adjustable, then a simple HVC circuit will do.

BMS is there to protect your pack when the primary control fails.

Balancing can be done separately

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serious_sam   1 kW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by serious_sam » Jul 29 2020 5:33am

goatman wrote:
Jul 28 2020 6:29pm
why not use the cheap bms and only balance charge to 4.2v when you think you need to, the rest of the time just charge to 4.1v
If you must use the non-programmable BMS, then I think this is the best option.

The cheapo non-programmable BMSs are set to balance at 4.2V (or very close to). So to balance you will need to fully charge and maintain some trickle for the balancing period.

But if your cells are well matched and in good condition, then you don't need to balance every charge anyway. I haven't looked into modifying the smaller ones (4S) that I use in electronic projects at home because that's what I do -> balance charge once every 5 or 10 charges.

If you have old or unmatched cells, then I would strongly suggest getting something more programmable and allows monitoring.

The problem I have with the non-programmable BMSs is that you have no indication of what is going on. You don't know if it's actually doing anything.

The best feature of the bluetooth BMS is being able to view all cell voltages on your phone. No need to access the cells and probe with a multimeter.

Tommm   10 kW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by Tommm » Jul 29 2020 6:51am

Changing your charger to only charge to 90% is easier. But even that is risky since cheapo bms might only balance at a high voltage.
Always get a bluetooth bms!!!

docw009   100 kW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by docw009 » Jul 29 2020 9:41am

Betcha most of your cheapo BMS don't even have balance circuits. All they do is shut off the battery below and above the cell set points.

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by Tommm » Jul 29 2020 9:48am

docw009 wrote:
Jul 29 2020 9:41am
Betcha most of your cheapo BMS don't even have balance circuits. All they do is shut off the battery below and above the cell set points.
After having my battery repaired (it was faulty bms, pack went out of balance), the battery repair guy said most repairs he does is due to crappy bms-es and sometime vibrations.

If you buy a bluetooth bms, when it starts talking nonsense or not doing its job you will know, even if the bms it isn't of higher quality, though usually they are a step above.

paka   1 µW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by paka » Jul 29 2020 10:48am

why not use the cheap bms and only balance charge to 4.2v when you think you need to, the rest of the time just charge to 4.1v
Would this approach work for e.g. whole 36V battery just by lowering the charge voltage from 42V to 41V ?

marka-ee   10 mW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by marka-ee » Jul 29 2020 3:26pm

paka wrote:
Jul 29 2020 10:48am
why not use the cheap bms and only balance charge to 4.2v when you think you need to, the rest of the time just charge to 4.1v
Would this approach work for e.g. whole 36V battery just by lowering the charge voltage from 42V to 41V ?
I'm doing that now, with my 10s packs. I used to charge them with my icharger that can to 10s balance charging, but something in it failed and it reports cells 7 to 10 as way off voltage. Calibration won't fix it. This is the second icharger I have had do this odd thing. I tried fixing it, but the circuit design is bizarre , the designer used some weird hacks to get the upper cells to read voltage.

So that's what I will do now, charge every once in a while with my ANT bms to let it balance. I sure wish these 'smart' bms could do more like 1 amp balance capabilities, but overnight should be ok.

marka-ee   10 mW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by marka-ee » Jul 29 2020 3:32pm

Balmorhea wrote:
Jul 28 2020 5:10pm
Yeah, who cares about having maximum range? We're just like stinking car drivers; we can carry as many thousands of pounds of batteries as we like.

If you want control over the BMS parameters, get a programmable BMS. Otherwise, expect to exercise your own discretion about how high to charge and how low to discharge.
I care about range, but charging to 4.1 probably doubles the life of the battery. The extra range gained by going to 4.2 is only 10 or 15 percent. Also I usually only discharge to 3.2 .

goatman   1 MW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by goatman » Jul 29 2020 11:08pm

you can charge to 4.15v

viewtopic.php?f=16&t=9268

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monster   100 kW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by monster » Aug 01 2020 9:21am

It's worse than that. The cheap bms just cut off when first cell reaches 4.3v. There is no cell balance.
They are just to prevent fire.

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fechter   100 GW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by fechter » Aug 01 2020 11:49am

marka-ee wrote:
Jul 28 2020 4:13pm
I have a couple of the el-cheapo-ebay-china BMS's that are design goal wise set to 4.2 for the balance voltages. I like to run my packs at a max of 4.1 V/cell to promote pack health. So maybe I can change it to 4.1 ? Maybe a bunch of resistor changes or something ? I'm just shooting in the dark. I do have one of the programmable 'ANT' type BMS also, but I'd like to get some use out of the more compact and cheaper 'dumb' BMS's as well. Side question : Why do they all seem to use 4.2 as max voltage ? Great way to shorten a packs life. Imagine if EV cars did something so dumb, always charge to the max cell rating.
All the ones I've used have balancing shunts but the only way to change the balancing voltage would be to replace all the voltage detector chips (one for each cell). They are microscopically tiny and you would need to find a 4.1v version that has the same pin out. Not really practical. I wish you could by them with 4.1v balancing shunts as this would be nearly ideal. You could still charge to 4.2v if you wanted to as long as you ride the bike immediately after coming off the charger.
"One test is worth a thousand opinions"

john61ct   100 GW

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Re: can I change a cheapo BMS to be 4.1 volts / cell?

Post by john61ct » Aug 01 2020 4:27pm

It's not like BMS that are adjustable are even that expensive

still a small fraction of what, you need to pay for quality, IOW accuracy reliability longevity.

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