Single eg4 6000 with two 48v batteries

[D. Abineri]

I currently have one 48v 200AH battery connected to my eg4. If I obtain another 48v battery at, say, 4Ah, can I just connect the batteries in parallel before the connection to the inverter? Is there a problem, in general, in connecting two such batteries in parallel?
Thanks for thoughts on this.

 

[sunshine_eggo]

I would not bother with connecting 4Ah to an existing 200Ah.

 

[Smorris998]

There’s a lot of complexities to this, but the core is, if they’re the same voltage, chemistry AND have a common port bms (1 connection point for in and out) then it’s okay. But if there is a bit discrepancy between capacities and current delivery ability it can lead to the smaller battery getting damaged over time as it is used (this is conjecture, but there is some good logical thought behind it). Considering how huge the discrepancy is here ( litterally it’s 2% the size of the main pack), it’s probably a reallly bad idea.

I have 12 48v ebike batteries In parallel that have varying capacities of 15-20ah and have been using it for several years without incident (but I have split port bms so seperate charging and discharging and have diodes that prevent current sharing between packs)

 

[timselectric]

I currently have one 48v 200AH battery connected to my eg4. If I obtain another 48v battery at, say, 4Ah, can I just connect the batteries in parallel before the connection to the inverter? Is there a problem, in general, in connecting two such batteries in parallel?
Thanks for thoughts on this.

It will work perfectly fine. But why bother with the 4ah battery at all?

 

[sunshine_eggo]

I am concerned that the massive disparity means the 4Ah battery could easily exceed it's C ratings. IIRC, Andy did some similar tests, but I can't be arsed.

 

[timselectric]

I am concerned that the massive disparity means the 4Ah battery could easily exceed it's C ratings. IIRC, Andy did some similar tests, but I can't be arsed.

Andy's test verified that it will work perfectly.
Voltage is the great balancer in parallel setups.
They will equally charge and discharge based on their percentage of the total capacity.

 

[Smorris998]

While it may be fine, but it’s a lot of what if to add 2% capacity. really not worth the risk. Also, since it’s 4ah it’s probably made from a123 or some other cell sized lfp that could have completely different IR values which would lead to current imbalances and possible overcurrent issues and huge balancing currents if someone went wrong .say it went into protect for something for like a hour, and the other pack gained a few volts from charging, once the protect turns off the current flow would be huge and could possible short the mosfets in the bms and then just basically make the cells ignite from overcurrent heating. This is an edge case obviously but considering we’re talking lithium batteries you have to consider this stuff.

 

[timselectric]

It's definitely not worth the effort, for 4ah of added capacity.