Adding a new, different size battery to my lifepo4 setup?

[Tasemu]

Hi all.

Up until recently I have been using a 280AH Lifepo4 bank on my narrow-boat with a Daly 250A BMS. Over a winter and a year or so, I noticed that I did not give the batteries time to balance and they had eventually gotten so out of balance that I felt the need to do another top balance. I took the opportunity to to purchase a new pre-made 200AH battery with an overkill bms from lifebatteries (UK) and installed it to take over the system while the top balance was being performed on the old bank.

Now I am wondering if it is possible to connect my old battery with the DalyBMS back into my system and have a total of 2 batteries supplying my system. These would be as follows:

1x LifePower 200AH Overkill BMS
1x DIY 280AH Daly BMS

In my head it feels that there should be no issue with these batteries working together, each behind their own BMS and both running through the same Victron Smart-Shunt to monitor the sum capacity.

Posting this to see if my thoughts are valid, or if there is any reason why this may be a bad idea. Thanks in advance for any and all advice!

 

[rickst29]

Yes, that should work fine. Each battery has a "-" terminal from the corresponding BMS, and those get connected into the "battery" side of the Victron Shunt. The "+" terminals hook together into a bus (12v or 24v, whatever you have). If the 2 pairs of terminals wires have equal resistance, roughly equal amounts of current will be drawn from each - although the tightness of your connections, the quality of bus bars between battery cells, the resistance of the BMS units, and the resistance of cells within the batteries may cause some degree of imbalance. Because the BMS units are different, their resistance might be dissimilar.

If you can obtain another amp meter (with shunt), you could put that shunt into the "-" battery terminal leg of one battery. It would slightly increase resistance on the current path through that battery, but it would allow you to monitor the power levels two batteries independently. (One directly from the new monitor, the other value obtained by subtracting the new monitor amp-hours used from the total). Or you could move the current Victron to handle just one battery, with the new meter on the other.

With that measurement capability in place: If you find one battery being "over-used" in comparison to the other, during periods of high current draw, you can lengthen one or both of the terminal wires on the over-used battery - increasing resistance and making it "less attractive", bringing utilization of the two battery packs more even (and in line with their capacities).