Battery balancer between 12v lead acids with heavy discharging

Got me thinking about my own 24v setup actually. Running two 12v lead acids in series and heavy discharge cycles are definitely where balancing becomes a real headache.

Had a similar situation a while back with my Multiplus and a pump that pulled serious current in short bursts. What I found is that even batteries that test identically will drift apart pretty quickly once you start hammering them like that. One cell always seems to cop more of the abuse.

A passive balancer is better than nothing but honestly, for heavy cycling I'd argue an active balancer is worth every penny. The passive ones just bleed off the higher voltage as heat — fine for maintenance but they struggle to keep up when discharge rates are aggressive and recovery times are short.

Anyone here actually running a dedicated balancer on a 24v lead acid bank with high current draws? Curious whether you've seen measurable improvement in battery longevity or whether it made sod all difference in practice.

Also worth flagging — if your batteries are mismatched in age or capacity even slightly, no balancer is going to fully compensate. Might be worth load testing both individually before assuming the balancer is the fix.

I eventually moved over to a Fogstar Drift LiFePO4 setup and the balancing headaches basically vanished, but I know not everyone's ready to make that jump cost-wise.

What's everyone's experience — are balancers genuinely pulling their weight on lead acid banks under stress, or is it more of a sticking plaster?

@Wayne1980 Series lead-acid balancing under heavy discharge is genuinely brutal on the weaker cell — it'll get hammered disproportionately every cycle.

Worth considering an active balancer rather than passive. I ran a Victron Battery Balancer on my shepherd's hut setup for about 18 months and it made a measurable difference to voltage divergence under load. Passive resistive balancers just bleed off energy as heat, which is wasteful when you're already running tight on capacity.

Key thing people miss: balancers only help during float/absorption, not mid-discharge. If your batteries are already mismatched, no balancer fixes that — you need to individually charge each 12v battery and check resting voltages before assuming the balancer will save you.

What's your discharge rate roughly? If you're pulling above 0.2C consistently, lead-acid chemistry itself becomes the limiting factor regardless of balancing strategy.

@HighlandExplorer is right, and honestly in my experience with my boat setup this is where lead acid really starts to show its age as a chemistry.

One thing worth considering — active balancers rather than passive ones make a meaningful difference under heavy discharge. Passive just bleeds off the stronger battery, which is wasteful when you're already working the bank hard.

That said, I eventually moved away from the series lead acid arrangement entirely on the boat. The monitoring overhead alone was becoming a part-time job. Fogstar Drift LiFePO4 sorted it, built-in BMS handles cell balancing properly.

If you're committed to keeping the lead acids though, Victron's battery balancer is probably the most straightforward option — plug and play for a 24v series setup and it does give you some protection against the weaker unit dragging everything down.

Been there with my motorhome — ran two 12v AGMs in series for years and watched one slowly murder the other under heavy inverter loads.

The maths is brutal: even a 5% capacity mismatch between your two batteries means the weaker one hits 100% DoD while the stronger one's still got juice left, every single cycle accelerating the imbalance further.

A Victron Battery Balancer is the obvious answer for 24v lead acid series strings — passive but surprisingly effective at keeping terminal voltages matched during charging phases. Won't help much during discharge though, @Wayne1980, which is where @HighlandExplorer's point really bites.

Honestly the real fix is matching battery internal resistance before you even wire them in series — a proper load tester will save you months of grief down the line.

Really useful thread this. One thing worth adding to what @HighlandExplorer and @VoltChloe have touched on — a passive balancer alone often isn't enough under heavy discharge conditions. The bleed current on most passive units is tiny compared to your actual load current, so it's essentially just watching the imbalance happen rather than correcting it meaningfully.

If you're serious about keeping those two 12v banks in step, an active balancer is worth the extra spend. They actually transfer energy between the batteries rather than just dumping it as heat. Brands like Heltec do decent units for not a lot of money.

That said, @Wayne1980 — what capacity are your batteries and what sort of discharge rates are you seeing? That'd help narrow down whether balancing is even your main problem or if one battery is already on its way out.

Great points all round here. One thing I'd add that nobody's mentioned yet — temperature can massively skew how mismatched your two batteries appear, especially under heavy discharge. If one's sitting in a warmer spot (near an inverter for instance), its internal resistance will look quite different to the cooler one, and a balancer can end up fighting a battle it can't really win. Worth making sure both batteries are in similar thermal conditions before concluding one's genuinely weaker. @FormerCop's point about one slowly killing the other is spot on — I've seen it happen surprisingly quickly once the imbalance gets a foothold. Regular individual voltage checks under load (not just resting voltage) will tell you far more than any top-level reading across the whole 24v bank.