PV Home Battery - Combining Battery chemistry - Page 1
Forum section: electronics / general

author: user325153
author_rank: Newbie
timestamp: July 22, 2022, 01:48:23 pm
content: Hey Guys,For my PV installation I make use of four lead acid batteries in series.I thought these lead acid batteries would be toast by now but they are still working fine, they charge/discharge a few killowatt-hours per day.I am making a battery wall with rows of 18650 fused cell holders and a bms as a battery upgrade.The lead acid batteries have a temp sensor plugged in and for the Li-ion there can be an rs485 connection+temp sensor to the inverter.I was thinking about an (auto) switch that can switch between battery systems with some sort of precharge, or a system that could combine the two batteries.Do you have any ideas or could you identify problems if i were to just connect the two in parallel.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 22, 2022, 06:29:00 pm
content: That's going to be difficult.Direct parallel connection is nope, because the lead pack would go up to something like 14.4V*4 = 57.6V, which would then be 4.43V/cell for lithium.14s would be the closest li-ion equivalent for 48V lead acid. Then 57.6V would be just 4.11V/cell. I could perhaps-maybe consider permanent parallel connection (with fusing of course) of the 48V lead acid and 14s li-ion packs. But it's still quite iffy. It's just that any other solution (than permanent parallel connection) is going to be complex and you likely get something seriously wrong.

author: user325153
author_rank: Newbie
timestamp: July 24, 2022, 03:19:28 am
content: Thanks for your reply, I can set the max voltage on the charger of the inverter, but maybe then the lead acid won't charge. I was also concerned about continuously bulk charging the lead-acid from the lithium. The bms will regulate current, under voltage protection, over voltage protection. The inverter will also have a max current. I will try to figure out the circuit one more time..I am not too happy with these lithium cells anyway, new ones are okay but i had ones of a good brand that went into thermal overload by simply charging them while following all the rules.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 24, 2022, 06:53:51 am
content: Quote from: user325153 on July 24, 2022, 03:19:28 amVery likely not. What BMS do you have? One that regulates current will be expensive and a very special snowflake.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 24, 2022, 02:52:01 pm
content: Probably the simplest would be to use a Schottky diode to provide a discharge path from the lower voltage battery to the higher voltage battery, with a LDO to set the charge voltage of the lower voltage battery. Note that lead acid likes to be near full charge while lithium likes to be near mid charge, so it probably would work better to have the lithium battery be of a slightly higher nominal voltage.

author: Seekonk
author_rank: Super Contributor
timestamp: July 24, 2022, 03:10:06 pm
content: I have just a 12v system and use that diode trick to charge a lipo4 battery for an independent system from a lead battery. That keeps me in the 13.3-4 range and occasionally let it rise up to balance. I don't feed back to the lead system but could add a couple diodes to do that.