How are 12v LiFePO4 batteries supposed to behave in parallel discharge? - Page 1
Forum section: electronics / beginners

author: e100
author_rank: Frequent Contributor
timestamp: September 08, 2025, 04:32:54 am
content: (I asked this is in general a few days ago but did not see any replies so I have rephrased it here.https://www.eevblog.com/forum/chat/algorithm-for-controlling-the-charge-mosfet-in-a-lifepo4-battery-bms/)By 12v battery I mean 4 cells with an integrated BMS that has no data comms with any other battery.As each BMS does it's own thing it's possible for each one in a parallel configuration to be charged to a different voltage, usually somewhere between 13.6 and 14.2 v.The internal circuitry of the BMS is usually two back to back MOSFETs in series, one of which is responsible for halting charging usually because the pack as a whole, or one of the cells has reached some threshold voltage.What's supposed to happen under discharge conditions when these batteries have different voltages? Does the one with the highest voltage stop the others from discharging until it's voltage has dropped 0.6v below the others. Do you get a weird oscillation where the batteries take turns in supplying all the current and trigger over current charge protection in the others due to the imbalance between the batteries and the behavior of the charge protection MOSFET and its diode drop when in the off state?

author: Marco
author_rank: Super Contributor
timestamp: September 08, 2025, 09:25:04 am
content: Don't charge/discharge them anywhere near the over/undervoltage cutoff of the BMS, problem solved.

author: Simmed
author_rank: Frequent Contributor
timestamp: September 08, 2025, 10:08:45 am
content: Quote from: e100 on September 08, 2025, 04:32:54 ami think some hobbyist buy battery batches and bin/check them for exact same voltageso the question i think should be = how different are your batteries ?Quotea buss bar could be 0.002R, and if cell diff is 10mV, it could be 5A ? a waste of powerfrom another chatter on the web this 1 saysQuoteso 0.1v could be a huge deal esp for huge // batteries(im not battery expert)

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 08, 2025, 10:23:01 am
content: If the batteries are in parallel, they will all naturally equalise to the same voltage.A BMS is needed for series strings so you don't force current into a full cell while charging a weaker cell waiting for it to catch up, or over discharge a weak cell while other stronger cells still have charge.In parallel, any weaker cells are supported by the rest and it all generally balances out.

author: Simmed
author_rank: Frequent Contributor
timestamp: September 08, 2025, 10:50:48 am
content: Quote from: TERRA Operative on September 08, 2025, 10:23:01 ami think so too

author: ME
author_rank: Frequent Contributor
timestamp: September 08, 2025, 11:17:26 am
content: lets hope your local fire brigade is near!.

author: paulca
author_rank: Super Contributor
timestamp: September 08, 2025, 11:19:58 am
content: The two packs will equalise.DO NOT connect them in parallel if they are significantly different voltages.You can do the maths on this. It's not as bad as you might expect. If you connect a pack at, say, 12.40V to a pack at 12.60V, current will rapidly flow from the higher to the lower. However, the elasticity of that voltage and the voltage drop in the cables will usually bring it to something tolerable by the BMS.It is however better to bring them within 100mV charge before paralleling them and only parallel "post BMS". Such that the protection mosfets can protect the battery and cables if it goes wrong.It still does not permit you to bring both packs up to 14.40V. Not easily. If you have no current limiting on the charger side when one pack hits a cell HVC it will disconnect that pack and the other one will have to take 100% of the charge current.If you cannot taper the total current flow, I would suggest not aiming for 14.40V, but something like 14.00V. Usually you can set the float trigger voltage on chargers. Set the "absorb time" low and target a low enough voltage to give you headroom in an "out of balance" "top of charge".As an example of crude tapering. I have 3 solar charge controllers. I stagger their absorb->float transition voltage. So when the pack gets to about 27.6V in my case controller 1 switches to 27.4V float (basically mute). At 28V the second controller switches to float. Leaving only the final 330W panel fighting with the balancer. This was not enough to reach 28.80V and perform a "top balance", so I introduced a proportional backoff algo on the final charge controller which is a reciprocal action off of "cell delta".

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 08, 2025, 11:57:55 am
content: To equalise the batteries before connecting them in parallel, first connect them through an automotive lightbulb like used for an indicator or tail light etc to limit inrush current.Measure voltage across the light bulb until the voltage is under 100mV as mentioned above then complete the direct connection.

author: xvr
author_rank: Frequent Contributor
timestamp: September 08, 2025, 12:47:09 pm
content: Parallel connection of batteries is possible for batteries in the same condition and from the same batch. In this configuration, the batteries will equalize their charge during the charge/discharge process.All *P factory-made batteries are manufactured in this manner.You can connect different batteries in parallel, after manually equalizing them, but the weaker ones will wear out during use (charge/discharge cycles).Or you can use a special BMS/charger for parallel connected batteries, but that's a completely different story. Personally, I've never seen such a BMS (only for series connected batteries).

author: Psi
author_rank: Super Contributor
timestamp: September 08, 2025, 12:48:07 pm
content: So you have two batteries, each with 4 cells in series with its own BMS, and you have the two of those batteries connected in parallel?I guess my first question is how the BMS works and how its expecting you to charge them. eg, charged all in series, or charged through a balance tap.You need to determine what the BMS can and can't do. It can always stop charging or discharging by disconnecting the load. But exactly how smart is it, can it burn off some/all charging energy going into one cell over another to self balance? Or can it do that through some internal DCDC. or can't it do that at all.If it can do that somehow then it should keep each cell in the pack in balance on its own.The fact the two batteries are in parallel means one can remove itself if needed without causing an issue. So that should be fine as long as you don't need the two in parallel to get enough instantaneous current.When they are in parallel the current comes from the one with the highest voltage, which quickly bring down the voltage until the other one can supply some current. So they naturally find some sort of a shared equilibrium, it's not always 50/50, the one with the higher capacity puts in proportionally more so that the state of discharge % can track together (since they are locked together voltage wise).When you first connect them together there is some uncontrolled charging between them. So as others have said, that can be an issue of the voltage is too different then the current can get too high.There can be some issue here if, for whatever reason, the BCM on one battery removes itself from the circuit, and then tries to come back online after the other one is now at a different state of charge. eg, one battery totally disconnects for a temp reason, or because only one cell is flat while the others are all good. And so now the other battery takes all the load and fully discharges all cells. You then connect a charger and the both switch back online but at quite different total pack voltages. Some BCMs might be able to handle that situation, but some can't

author: e100
author_rank: Frequent Contributor
timestamp: September 08, 2025, 01:09:58 pm
content: Some more information. Despite being connected in parallel, in practice batteries don't equalize at the top end of the charge curve. This is caused by cells within a single battery being out of balance with respect to each other which causes the BMS to prematurely terminate the charging of the whole battery when one cell hits 3.65v.The poorer the balance the lower the voltage at which the BMS terminates charging. This means that each battery in a parallel configuration will terminate charging at a different voltage. In addition, the passive balancer circuits in cheap batteries are incapable of bringing the low and high cells into balance so it is common to see a 150mV differential between the highest and lowest cells within a single battery.I'm interested in understanding the discharge process when you have two or more batteries in parallel at different voltages in a state where the charge MOSFET has been disabled and therefore there is a diode drop at the battery terminals. Is it actually possible to get an even current draw from all batteries or will there always be an oscillation when the charge MOSFET of a particular battery gets enabled by the BMS and the diode drop disappears and the voltage difference between the batteries is exposed.

author: Psi
author_rank: Super Contributor
timestamp: September 08, 2025, 01:16:46 pm
content: Quote from: e100 on September 08, 2025, 01:09:58 pmI don't think you get a diode drop due to the back to back series mosfets in opposite directions required for proper on/off control.But i cant say I've looked into BMS's that closely.

author: paulca
author_rank: Super Contributor
timestamp: September 08, 2025, 02:08:31 pm
content: Quote from: Psi on September 08, 2025, 01:16:46 pmThat is how my BMS's work. A long row of parallel series mosfets on a heatsink. There isn't a voltage drop or the mosfets would melt. They tend to fail when breaking current overloads.BMS balancers are a mixed bag, but properly powerful ones are rare and expensive. I use two together. The BMS (JKBMS) fixates on 2 cells leaving other worse cells unattended. So I added a flying capacitor bank which migrates around the cell pairs.Still, you cannot expect to pump current into them when they get into the voltage "hockey stick region".When in the "bulk" charge voltage window of < 3.4VCell you can hit the with 1C or 2C current and the voltage will barely move. Above 3.4VCell the voltage rises much faster for a given input current.To fight this a BMS has to "burn off" or migrate charge from the high cells, to allow the lower cells to continue to charge. If you look at the spec on BMS balancers they often give a maximum cell balancing rate. 5W, 20W, 50W maybe if you spend money, but none of them will match a full kW of solar pumping into the cells when they go over 3.4V.You have a few options:1. Let the BMS cut the charge, it might upset your charge controller and can upset connected inverters. It is not ideal as it will burn you BMS mosfets out and if they fail short, your charge protection fails and thus your batteries vent. Relying on the last line of defense only is not wise.2. Cut the charge off much lower. Configure your charger to absorb or finish at a lower voltage, like 13.60V - you loose about 5% max.3. Use a charger or combination of devices to "taper" the charge power based on the max cell voltage/delta.You can do a hybrid approach, which many people do. Keep the standard charge voltage low, like 13.4 or 13.6. Then periodically, under supervision, use an actual charger to top balance them, or a load to bottom balance them.If it's an accessible pack, you can use a bench power supply across cells to top balance them.