Parallel battery puzzle - Page 1
Forum section: electronics / projects

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 11, 2025, 07:59:53 am
content: Context, think boat/motorhome leisure battery.In my case Lead-acid, theoretically this would be the same for LFP, except LFP probably has a BMS on every battery anyway.So, common wisdom is you need to connect them cross over.Like this:But what if you have 3 (or even 4) batteries parallel(lets not distract ourselves with a discussion about 24V series being better)We just extent it right? After all, what matters is that each path has the same length of wire resistances in it.And in fact, this indeed how may youtube UK narrowboat owners show 3 batteries being connected.And even Victron shows it like that:Lets simulate this. Check below picture and guess what happens, before clickingthis simulation.Maybe it is just me, but it surely did not match my expectations.(typical internal resistance 5mΩ, cable resistance 1mΩ, added to the simulation)I actually stumbled on this because I thought I knew how it worked and tried to show my brother who bought a narrowboat and asked for my help with the electrics.Maybe its just me, I probably am less smart than I think, but please let me know if this was instantly obvious to you.If so, please also let me know how you would correct for it.(it may not need correcting for small currents, but once you start extending this to 100A or 200A invertor peak loads, it does start to hit those outer batteries)Eventually I figured it out. It is all basic resistor 101. But still, it caught me off-guardHere is thelink with my explanation(with exaggerated wire resistances), andhow you would correctfor it.But I ask you to not click it if you want to solve the puzzle yourselfOh, and I have to hand it to Victron: They do show them in parallel without said correction, but they also do show some nice beefy over the top busbars. They know.

author: Fryguy
author_rank: Regular Contributor
timestamp: February 11, 2025, 08:25:52 am
content: This is no puzzle to me - if you use only single digit currents or low 2-digit , nothing serious will happen . But when you go to the higher 2-digit or 3-digit currents , cable resistance will matter .And the cross connection only works perfect for 2 batteries - if you use 3 or more batteries i recommend a star shaped distribution with identical cables to each battery .

author: Siwastaja
author_rank: Super Contributor
timestamp: February 11, 2025, 08:31:24 am
content: Connecting wiring is easy to make thick enough. Sure enough, at very high currents the batteries do not perfectly share the current, so some discharge/charge more than others. But then when the current drops, they equalize in charge again.And if you are all the time running high enough current that this balancing becomes an issue, you have probably much bigger issues - e.g. with wires heating up too much.Paralleling battery packs, especially li-ion packs, with "whatever" BMS, will come with much bigger can of worms than this. For example, BMS should be designed to disconnect at overload. Then it does a reconnect. If this reconnect is done at different SoC%, massive overcurrent peak happens. In thebest case, BMS can handle it and disconnect, so you just end up with completely nonfunctional system. In worst case, ...Parallel packs really require a special BMS system designed for paralleled packs, handling the initial pack balancing before (re)connection. Or doing the same manually, which then requires understanding and care.Really the big issue is asymmetrical maximum safe discharge vs charge current of li-ion batteries. Even if the BMS is able to handle short circuiting the pack gracefully, connecting two batteries with vastly different SoC in parallel is worse than short circuit: now the emptier battery sees a largechargingcurrent, not discharging current as in short circuit. Hopefully your BMS also understands this charge current limit and handles it separately.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 11, 2025, 09:02:15 am
content: Quote from: Fryguy on February 11, 2025, 08:25:52 amYes, I did find that solution too. It is the best, especially when you consider extra resistances on wire lugs, which cannot be simply fixed by doubling some wire lengths.But is also a bit ugly and unwieldy in small spaces.Arguably the Victron busbar image can be considered a star topology, be it with a very elongated star center.Quote from: Siwastaja on February 11, 2025, 08:31:24 amYeah, one of the reasons we do not use LFP. The other beeing we stay on the cheap for now, and the 3rd being theinsurance has some clausesabout LFP banks due to the fire risks.

author: radiolistener
author_rank: Super Contributor
timestamp: February 11, 2025, 10:11:25 am
content: Quote from: cybermaus on February 11, 2025, 07:59:53 amThe result is pretty obvious and predictable. Could you please explain what is surprising for you?At first I thought that you were surprised by the presence of resistance in the wires, but from the context of the message it seems that you understand this, so it is not clear - what exactly confused you?

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 11, 2025, 10:38:48 am
content: Some extra ref material:Victron:The_Wiring_Unlimited_book

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 11, 2025, 02:20:17 pm
content: Quote from: radiolistener on February 11, 2025, 10:11:25 amWhen calculating them as resistor networks, it does work out.But when practically working with batteries, no, it was not intuitively clear (to me) option A works perfect, even theoretically, but option B has issues.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 11, 2025, 03:32:48 pm
content: I've run lead acid batteries in parallel for a narrow boat, and I can say for sure that three or four in parallel works just fine. Provided the cables are plenty thick you can wire them as in drawing B above without problems.When doing the initial connecting up you should make sure they are all fully charged, or else a lot of current will flow from the batteries with a higher terminal voltage to the lower ones.It is true that some batteries might provide more current than others if you mix different ages and types, but generally speaking it doesn't matter and the batteries will sort themselves when they are next charged.Note that in the diagram shown by@cybermaus, all four arrangements are electrically identical if you use thick enough cable.So:1/ Use identical batteries (age, brand, Ah rating) if at all possible2/ Fully charge them all individually before connecting together3/ Use really thick cable to connect them together (like car starter motor cable)I've never used LiFePO4 batteries in a boat, so cannot comment.

author: Peabody
author_rank: Super Contributor
timestamp: February 11, 2025, 04:47:00 pm
content: So then for lithium ion, is it safe to use parallel 18650s for example? My assumption has been that you can, provided they are essentially the same voltage when first connected, and are never disconnected after that. But that means they can't be protected batteries because protection would kick in at different points for each battery, and there might be hell to pay when all are reconnected. So protection would have to be all-or-none. But in forums I have seen EEs say to never parallel lithium ion/poly cells.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 11, 2025, 05:16:11 pm
content: Quote from: Peabody on February 11, 2025, 04:47:00 pmTheory is, when using more then 2 in parallel, the outer ones will drain faster.Practically, everyone has been doing it for years, so unless when using high-drain deep-discharge situations, it is probably marginal and ignorable.Or maybe not. Maybe this is why packs always seem to wear out faster then desirable, but everyone just curses, shrugs shoulders, and buys a new one.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 11, 2025, 07:00:44 pm
content: Quote from: Peabody on February 11, 2025, 04:47:00 pmThey are designed to be paralleled. It is normal practice to thus parallel them. It's their intended use. For example, 5..6Ah 18V power tool packs use two 18650's in parallel, and five of those groups in series.You of course need to understand what you are doing - that includes verification of initial balance, making sure the paralleling connections can't become loose by accident (i.e., use permanent spot welding), and possibly failure analysis of PTC/CID devices of those cells. 18650 li-ion cells are specialty components, not meant for consumers/hobbyists/laymen.Paralleled cells are treated like "one bigger cell".Paralleling packs that are series-first is much more complicated (each pack requiring its own BMS), which is why normally you should do parallel-first pack, which is just... a bigger pack, not much different than using larger cells instead.