Charger considerations for two 36v LiFePO4 batteries in series

[mbesto]

I just bought two 36v batteries from Li Time.

LiTime 36V 100Ah Lithium Golf Cart Batteries for Club Car

According to the charging spec:

• Charge Method：
  • CC/CV
• Charge Voltage：
  • 43.2V±0.6V
• Recommend Charge Current：
  • 20A(0.2C)

I'm putting them in series to create a 72v battery for a golf cart. Based on the spec, I believe this means I can get a VDC charger of anywhere from 85.2V to 87.6V (86.4 ideal)

Few questions:

1. What charger should I get? I'm looking at the following:
   1. 72V 15A Charger (86.4V DC) – Ring Terminals (Big Battery)
   2. 72V 24S 87.6V lifepo (Ali Express) ​
2. What happens once a LiFePO4 battery is fully charged? Does the charger float?
3. Is an active balancer necessary?

 

[sunshine_eggo]

Few questions:

1. What charger should I get? I'm looking at the following:
   1. 72V 15A Charger (86.4V DC) – Ring Terminals (Big Battery)
   2. 72V 24S 87.6V lifepo (Ali Express) ​
2. What happens once a LiFePO4 battery is fully charged? Does the charger float?
3. Is an active balancer necessary?

1. Whichever you prefer, but I would prefer 82.8V for longer cycle life.
2. Float is a charger function not battery, and the desire to float a LFP battery depends on the application. For a golf cart, I would say no, but you do want it to hold peak voltage for 30-60 minutes at 86.4 or 2 hours at 82.8V to ensure it's topped off and balanced.
3. Would be preferred, but I'm not aware of any for a 72V/36V battery.

 

[mbesto]

Got it. Thanks. I just got a support message from Li Time saying that they actually don't support that battery model in series (not entirely sure why honestly). Sounds like this is due to charging. Doing some quick internet searches it sounds like the only solution would be to charge each battery independently, which doesn't seem worth the trouble.

 

[sunshine_eggo]

The BMS of each battery must be able to tolerate the voltage of the entire string voltage. Many do not or are limited to 48V.

 

[hwy17]

Can you mount the charger in the golf cart? I think 2x 36v chargers would be most convenient and keep the batteries balanced.

You could use 1 36v charger but then you have to move it around or reconfigure the batteries in parallel to charge. 2 chargers could be permanently wired in and then you'd just plug both in to charge it. 10A chargers could be combined into 1 plug, or 30A chargers could be combined into a 20 amp plug maybe.

 

[mbesto]

Can you mount the charger in the golf cart? I think 2x 36v chargers would be most convenient and keep the batteries balanced.

You could use 1 36v charger but then you have to move it around or reconfigure the batteries in parallel to charge. 2 chargers could be permanently wired in and then you'd just plug both in to charge it. 10A chargers could be combined into 1 plug, or 30A chargers could be combined into a 20 amp plug maybe.

absolutely - i would probably just get two chargers and charge both at the same time, which wouldn't be a big deal honestly. still waiting to hear the reply from Li Time. My only worry is that they discharge at different rates, which would be bad.

 

[mbesto]

The BMS of each battery must be able to tolerate the voltage of the entire string voltage. Many do not or are limited to 48V.

Got it. Ya that's likely the culprit. I'll wait to confirm from Li Time.

 

[mbesto]

Dang, this was the result I got from Li Time:

Unfortunately it's not the charging that's the problem. It's due to a technical issue, the board for 36V 100Ah batteries doesn't support 2 identical 36V batteries in series.
Our batteries cannot be connected in series to make the battery system more than 48V.
For example:
A 12V battery in series supports up to 4 batteries (12*4=48V).
24V batteries in series support up to 2 (24*2=48V).
36V/48V batteries do not support series connection.

So pls do not connect 2*36V batteries in series. Hope you can use our battery properly.

 

[hwy17]

I think the chances of the BMS frying in series is low. But it's true it does expose them to full system voltage potentials particularly when they enter discharge protection.

And that's I think the most likely problem that you'll face is the batteries entering overload discharge protection when you gas it from a stop.

So depending on your risk tolerance either have to back out of the plan completely, or go ahead and try.