Hi everyone. I have a few questions regarding converting my batteries from a 12 volt to a 24 volt system. My plan is to convert to 24 volt so I can double up on my panels without having to replace my charge controller. I’m using a victron 100/50 that I already have maxed out using a 12 volt system. I currently have 2) 100ah battle born batteries in parallel. I guess my question is am I stuck with battle born batteries? I’d really like to build my own 24 volt battery with raw cells but then what do I do with the battle borns? Is it possible to put the battleborns in series (for a 24v battery) and then parallel them to the 24 volt raw cell battery? Any help would be appreciated, thanks!
Converting to 24 volt
- Thread starter Mike7599
- Start date
BentleyJ
Solar Wizard
This question has been asked a few times lately, guess everyone is up-sizing. Generally speaking its OK to mix different Brands of battery when connecting them in parallel as long as they are the same type of chemistry. In this case LiFePO4.
My 48V system has 2 different LiFePO4 batteries in parallel and they work just fine. In fact, they are not even the same Ah rating. One is 150Ah while the other is a DIY 120Ah. What happens is that they self balance during the charge and discharge process. This is because any small voltage imbalance that develops between batteries will cause the current to favor the battery that needs, or can deliver, more or less current.
My 48V system has 2 different LiFePO4 batteries in parallel and they work just fine. In fact, they are not even the same Ah rating. One is 150Ah while the other is a DIY 120Ah. What happens is that they self balance during the charge and discharge process. This is because any small voltage imbalance that develops between batteries will cause the current to favor the battery that needs, or can deliver, more or less current.
Thanks for your reply. A recent video by Will Prowse suggested putting a breaker on each battery if using different manufacturers. Did you do this with your batteries? If so how did you size up the breaker? Do you match it up to the BMS’ max output amps? Thanks
BentleyJ
Solar Wizard
Typically each stand alone battery should have a means of over current protection (Breakers or fuses will work, I prefer breakers.) and some form of BMS. The 150Ah battery is a YIYEN Brand sourced on Alibaba. It is self contained and has built in BMS and Over Current Protection. The 120Ah battery, I used a 100A marine style Blue Sea lever type breaker (7044 187 series). There is some disagreement about this breaker being suitable for LFP 48V batteries due to the Actual voltages being as high as 58V. In your case its a 24V system and would be well under the stated 48V max range.
When choosing fuses or breakers they have to be sized based on the max. current carrying capacity of the conductors or can be downsized to protect the battery if its maximum discharge rate is less than the wiring. In no case should they be up-sized beyond the rating of the lowest current carrying device in the system.
When choosing fuses or breakers they have to be sized based on the max. current carrying capacity of the conductors or can be downsized to protect the battery if its maximum discharge rate is less than the wiring. In no case should they be up-sized beyond the rating of the lowest current carrying device in the system.
Thank you for the great info. So if I built a 24 volt battery using raw cells that had a 100 amp bms and was using 2 gauge wire I’d be safe using a 100 amp breaker? Also my battleborn data sheet says it’s rated for 100a continuous use. I assume this means it has a 100 amp bms? So in theory if I paralleled the 24 volt battleborns with the 24 volt raw cell battery would the combined bms current capability double to 200 amps?
BentleyJ
Solar Wizard
Yes, when batteries are in parallel the voltage stays the same and each can contribute its maximum current to the load. 2 x 100A = 200A.
One additional comment. Most of the popular value priced BMS products from China have what I consider to be somewhat undersized wire for their CLAIMED continuous current. If you expect your DIY battery to supply 75Amps or more quite often, I would up size the BMS to the next size available by that manufacturer. You are not forced to use the capacity of the BMS, you can specify the maximum Charge and Discharge current that the BMS will cut-out.
Thank you again for your help. I’ve had problems tracking down a reliable 24v bms with more than 100a capability. Do you have any recommendations that can be sourced from inside the United States? I’ve also read it’s possible to parallel two bms’ to one battery?
BentleyJ
Solar Wizard
There several BMS choices. I personally don't like the all in one type where they have PCB mounded power FET's. Unfortunately that style seems to be the most popular. Even if you source the BMS from US stock it probably going to be from China anyway. I cannot make any specific recommendations as I have not used any of these myself. Here is a list that I'm aware of.
Daly
Chargery
Current Connected
Overkill Solar
ElectroDacus
JK BMS
QUCC
Batrium
GWL123
Daly
Chargery
Current Connected
Overkill Solar
ElectroDacus
JK BMS
QUCC
Batrium
GWL123
Bossrox
Solar tinkerer
Yes you can series the battleborn's & connect them to lead/acid's. I'm doing it & they're working great together but I'm using the user mode on the charger & limiting the absorption voltage to 27.8v 27.6 for float. It takes longer for the lead/acid's to charge but maintaining a 3.45 max per cell on the lith's is most important. The thing I like about that combo is the lead/acid's don't kick in till the lith's drop to 25.5v so as the lith's are nearing the end cycle, the sla's are just kicking in slowing the lith's discharge speed. Another plus is when the bms shuts down you don't lose the inverter & have a nearly fully charged sla taking over.
