Battery Balancer rapid alarm on/off switching

Ran into something similar with my cabin setup a while back and never fully got to the bottom of it.

Had two 12v AGMs wired in series for a 24v bank, with a Victron Battery Balancer sitting between them. Everything looked fine on paper but the balancer LED was flickering on/off rapidly — not a steady alarm, just this frantic switching that made me think something was genuinely wrong.

Turned out one battery was noticeably weaker than the other (different ages, my fault for mixing them). The balancer was basically fighting itself trying to compensate for a cell difference that was too far gone to fix properly.

Few things worth checking if you're seeing the same:

• Are both batteries from the same batch/age? Mixing old and new in series is asking for grief
• Check individual battery voltages under load — if they're diverging by more than ~0.1v regularly, something's off
• Victron's own docs suggest the balancer LED behaviour can indicate the balancer is actually working hard, not necessarily failing — but rapid on/off is a bit different to a steady indication

With a 2kw array through an MPPT 250/85 you've got plenty of charge coming in, so it's probably not a charge shortage issue. More likely a battery mismatch.

Anyone else had this with Renogy batteries specifically? Curious whether it's a quality consistency thing or just bad luck. My cabin's now on Fogstar lithium and the headaches basically disappeared overnight.

@GlenWard the rapid cycling alarm is almost always the balancer hunting — it's detecting a voltage differential, firing current across to correct it, overshooting, then triggering again in the opposite direction. Classic oscillation.

Few things worth checking:

• Battery impedance mismatch — if one AGM is significantly more degraded than the other, the balancer simply can't stabilise them. It'll chase its own tail indefinitely.
• Load/charge fluctuations — even small ripple from a charger can confuse the balancer's threshold detection at 24v.
• Wiring resistance asymmetry — unequal cable lengths between cells and the balancer midpoint connection will skew the readings.

Had identical behaviour with two Yuasa AGMs before I moved to a matched Fogstar lithium bank. Ultimately the batteries were just too far gone for the balancer to cope with.

What's the age difference between your two AGMs? That's usually the root cause.

@RetiredSquaddie is right about the hunting, but worth checking one thing that catches people out — if either of your AGMs has a weak cell, the balancer will never settle because it's chasing a moving target. The differential keeps shifting rather than converging.

Quick test: fully charge both batteries individually with a proper charger, then measure resting voltage on each after 4-6 hours off load. If they're not within about 20mV of each other, you've likely got a duff cell somewhere.

Had exactly this on my narrowboat with two 110Ah AGMs. Balancer was going mad. Turned out one battery was genuinely dying — new matched pair sorted it completely.

Also worth confirming your balancer firmware is current if it's a Victron unit — there were some known hunting issues in older versions.

@PennineNomad curious what you were about to say there — your post cuts off mid-sentence, classic forum timing!

What I'll add from my own experience: the alarm cycling was far worse in my cabin when the bank was sitting at a partial state of charge for extended periods. The balancer seemed to go almost frantic around the 23-24v range, constantly correcting tiny differentials that probably didn't matter in practice.

Once I sorted the charging profile properly through my Victron MPPT and made sure the bank was actually reaching full absorption regularly, the hunting calmed right down. The two AGMs were never quite matched from the start — bought at different times, which I'd not recommend to anyone now.

If your AGMs have genuinely drifted apart in capacity, no balancer will mask that forever. At some point you're just delaying the inevitable replacement.

@PennineNomad's cut-off post was probably heading somewhere important — I'd bet they were about to mention internal resistance mismatch.

Had this exact hunting behaviour on my motorhome build years ago. Two AGMs, looked identical on paper, but one was a couple of seasons older. The balancer kept seeing a differential, correcting it, overshooting, then correcting again — alarm cycling every few seconds.

The fix that actually worked for me: load test each battery individually. Voltage at rest means nothing; it's how they behave under load that tells the real story. Mine showed a significant resistance difference, which explained everything.

Worth also checking your inter-battery cabling is truly identical in length and gauge — any resistance difference there creates an artificial imbalance the balancer can't distinguish from a genuine cell issue.

Mismatched AGMs in series is rarely a balancer problem — it's almost always a battery problem.