Do I need a BMS

[jdieringer]

Hi all, found this great community through watching Will on the youtube channel. My use case is a little different from others so I am curious thoughts as to is a BMS needed. I fly R/C airplanes and used to charge my batteries with my Prius, it was nice as it had 70A of 12V on tap when the ignition was on. Being a hybrid, the ICE would only pop on every once in a while.

I now have a minivan. I am not really keen on getting a portable inverter generator but would rather like to build a 4s 280Ah pack to take on the road. My math says I will get 12 or so charges of my flight packs which is plenty for a weekend of flying at contests. I do not intend to connect the battery to my vehicle, but would rather charge and store offline. I am battling if I need a BMS. I have the following:

iCharger 4010Duo which in synchronous mode can balance charge the LiFePO4 4s 280Ah battery at 70A with 2.4A sink current per balance tap. I power this with a Meanwell 1000W 24V DC power supply so should be about right. I also have an icharger 308 Duo which can do the same but 50A synchronous.

The iCharger can charge with or without balance lead so I can use a BMS. 99% of the time, I will top balance the pack, go to a contest, charge flight batteries to maybe 20-50% of the big battery and then come home. If I am going to be putting it away for a while, I may storage charge it to 50%. The chargers do have capability to discharge packs back into the source (regenerative discharge) but that will not be balance assisted without a BMS. The next charge cycle will be balance charged though.

So I see 2 scenarios.
1: Use a BMS and use my charger as a DC DC charger. The BMS would then continuously balance my big battery and also handle balancing of my regenerative discharge cycles.
2: Forget the BMS, it won't get that far out of sync in a couple cycles and rely on my charger to top balance.

I will also probably install a shunt to track the charge state.
Thanks!

 

[snoobler]

Welcome to the forum.

Those Duos are nice units. I have a handful of 3010B and a few dozen 206B.

1. You don't want a BMS balancing all the time. You want it balancing only during charge and above 3.4V/cell. Otherwise, you'll undo a top balance.
2. You have charging covered, but you don't have discharging covered. Risk of damaging lowest cell via a deep discharge.

 

[jdieringer]

I love these chargers. I know they have the new X and S series, but I really like the user interface (button and knobs) of the duos.

Thanks for that information, something I missed that the BMS only balances at 3.4V plus.

On the icharger, I can set the DC source profile as battery and put a lower source limit, but I know that doesn't guarantee catching a single cell runaway. If I am balance charging from my icharger, I will have JST-XH connector exposed to go to check balance, but I am guessing this issue will only really present itself at the bottom 20% as the discharge of LiFePO4 is pretty flat.

Thanks for your help!

 

[jdieringer]

I guess I have also been operating under the assumption that the iCharger with balancing and the BMS will conflict. Maybe that is a poor assumption and I can happily balance charge the pack with my iCharger and have the BMS connected to handle low voltage situations. Something like the Daly 100A with 50A charge current aligns pretty well with my use case.

 

[snoobler]

I love these chargers. I know they have the new X and S series, but I really like the user interface (button and knobs) of the duos.

Thanks for that information, something I missed that the BMS only balances at 3.4V plus.

On the icharger, I can set the DC source profile as battery and put a lower source limit, but I know that doesn't guarantee catching a single cell runaway. If I am balance charging from my icharger, I will have JST-XH connector exposed to go to check balance, but I am guessing this issue will only really present itself at the bottom 20% as the discharge of LiFePO4 is pretty flat.

Thanks for your help!

It doesn't only do that. It needs to be setup to do that. Many just balance at any point based on a voltage deviation.

I guess I have also been operating under the assumption that the iCharger with balancing and the BMS will conflict. Maybe that is a poor assumption and I can happily balance charge the pack with my iCharger and have the BMS connected to handle low voltage situations. Something like the Daly 100A with 50A charge current aligns pretty well with my use case.

They will not conflict. Your typical BMS balance is resistor bleed-off based and XXmA. Your balance charger will laugh at it as it stomps all over it.

 

[jdieringer]

snoobler, thanks so much. It sounds like 4s 280Ah with an overkill solar BMS (I like you can switch it on and off and it can do 120A so oversized for me), and a whatever shunt based capacity monitor will be a solid setup to get me started. When charging I can balance charge with my icharger as if it were a normal 4S LiFePO4 battery but have the piece of mind that it is protected. I can also set the profile of the charger to monitor as a first pass device disconnect leaving the BMS as a safety function. I think I am getting it a little clearer in my head. What a huge help!

 

[jharkin]

Another new guy here that found this place for the same reason... building a LFP to replace the heavy AGM I used for field charging.

I’m almost done with a similar project, but smaller... 90Ah cells and I use a PL6 (my biggest electrics are 6S3700s, rest gas) I did opt for the BMS just in case, but I’m disabling BMS balance altogether and just using the powerlab to balance. The Icharger if it’s anything like the PL will make bms balancing pointless...Our chargers can balance throughout the entire charge and 1-2A active balancing vs. the tiny passive balancing current of most of these BMS..

Ive run a couple cycles and watched the graphs..lt works great, go for it.

BTW, what are you flying? Your numbers make me think you are flying 12S5000s so I’m guessing 700 Heli or 90in+ IMAC?

 

[jdieringer]

Awesome, thanks for the input. I fly F3A so charging 10S 5200-6000mah packs. That's why the iCharger 4010Duo is ideal as I can charge 2 flight packs at a time without any paracharging. (A lot of guys will use a 6 cell charger and put the 2 batteries in parallel to charge.)

My batteries are on the boat, will let you know in a couple months when they come .

 

[jharkin]

BTW, I did the "parallel top balance" the way its reccomended here and it took FOREVER as the PL limted to 20A in unbalanced mode. I should have just let it balance normally the first time.

If your curious this is what a partial charge looked like. Starting at about 60% charged, 34A (.3C) CC to 3.6vpc C/10 (3.4A) CV. The cells stayed really tight all the way to 3.5 and only then started to diverge. It didn't completely balance only because I set the profile to timeout in 15min. I could set a long timeout and it would get them tighter but in practice I probably wont bother, that last 1% of capacity isn't necessary.

My cells where cheap AliE (from Wills list) and even though labelled grade A one cell is definately weaker than the rest. You can see it in the IR graphs as well and heat gun shows it warms a couple degrees more when charging. Still dont really care if I only get 10 years out of them rather than 15 .....

 

[jharkin]

Awesome, thanks for the input. I fly F3A so charging 10S 5200-6000mah packs. That's why the iCharger 4010Duo is ideal as I can charge 2 flight packs at a time without any paracharging. (A lot of guys will use a 6 cell charger and put the 2 batteries in parallel to charge.)

My batteries are on the boat, will let you know in a couple months when they come .

Aha.. I guessed close...

Duos are nice. Ive got my eye on the new Dual PL6 touch. Just like htat ecosystem as Ive been using them since they first supported A213s back in the 00's.... Thinking dual not because of big packs like yours, but having LOTS of small 3S2000/3000s and 6S1500s for foamies. Being able to parallel charge 8 or more of them at a time would make it easier to run out on short notice....

 

[jdieringer]

So some positive news. I built my battery and it is working great. Flew all day yesterday and charged maybe 7-8 packs and came home with 70% still in the battery (I did regen discharge a couple packs I didn't use back into the LiFePO4 battery, which is also a nice feature!).

I think the answer to this is no, but any issue with using two balance chargers in parallel to charge the LiFePO4 battery? The chargers in question are two separate iCharger x12's. My concern is that any small calibration issue would cause the balance portion to oscillate between the chargers. I can make one terminate on balance and the other terminate at like 50% of charge current so only one is really working at the end of the charge. Thoughts?