Set the floor where morning is soon enough

The bank crossed your critical threshold at ten past two in the morning. Quiet hours held the message, as they hold every message that is not a safety alarm, and you read it at seven with the fridge already warm and the inverter off. The alert worked exactly as designed. The threshold was in the wrong place.

That is the whole of this article, really. A battery alert is not like a leak alert, where the number is dictated by physics. Where you put a battery floor is a decision about when you want to be told, and almost everybody sets it too low the first time, because they set it where the battery is in trouble rather than where they still have options.

The battery monitor guide is the argument about which monitor to own. This one assumes you have one and answers the two questions that follow: where the threshold goes, and how much time it actually buys you.

Where to put the floor

Set it high enough that being told in the morning is still soon enough to act. That single rule handles the quiet-hours problem, the “I was driving” problem and the “I was up a mountain with no signal” problem all at once, and it is why every number below is well above where the bank is actually in danger.

On a shunt, by state of charge

Examples, and they differ by chemistry rather than by taste. Lead-acid: warn below 55 per cent, critical below 40. Lead-acid should not routinely go under half, so a warning at 55 arrives while you still have a normal amount of battery. LiFePO4: warn below 30 per cent, critical below 15, both comfortably above where a BMS disconnects and takes the decision away from you.

A SmartShunt reads every 60 seconds and its percentage is the only number here that is counted rather than inferred, which is why it gets the primary rule and the voltage gets the backstop.

On anything else, by voltage

For a resting 12 volt lead-acid bank: warn below 12.2 volts, roughly half, and critical below 11.9. On LiFePO4: warn below 12.9 and critical below 12.4, which is the top of the cliff rather than partway down it.

Two adjustments worth making. A Lancol reads every 120 seconds and gives voltage only, which is enough for a lead-acid bank and nearly blind on lithium. And a SeeLevel board already reports a voltage alongside its tank levels, which makes it a free second opinion if the coach came with one — worth having precisely because it fails differently from the monitor at the terminals.

How long that leaves you

Nothing in the catalog broadcasts a time-to-empty, and the app alerts on a level rather than a rate, so hours-remaining is a sum you do yourself from two numbers already on the dashboard. It takes about ten seconds and it is the difference between “the battery is low” and “I have until lunchtime”.

Usable amp-hours is roughly your capacity multiplied by the gap between the current state of charge and your floor. Hours is that divided by the current you are drawing. So a 200 amp-hour LiFePO4 bank sitting at 40 per cent, with a floor at 15, has about 50 amp-hours to spend; at a 5 amp draw that is around ten hours. Use a 50 per cent floor for the same sum on lead-acid, and the answer will be shorter than people expect.

Two readings, one multiplication, one division. It is worth doing once on a quiet evening simply to learn what your own rig’s numbers look like, because the figure you get is usually either much better or much worse than the one you had in your head.

Why a confident percentage can be wrong

A shunt does not measure state of charge. It counts what goes in and out and keeps a running total, which means the number is only as good as its starting point and its arithmetic. Three ways that goes wrong, all of them quiet:

• It has never been synchronized to a full charge. Until it sees the bank genuinely full, it is reporting a number it was told rather than one it worked out. After a reset it reads whatever it was given.
• The configured capacity is wrong. A shunt set up for the bank you replaced is confidently, consistently, proportionally wrong.
• Something is wired around it. Only current passing through the shunt gets counted, and factory inverter installs quite often land straight on the battery post. That load is invisible, and it drags the percentage off over days rather than announcing itself.

This is the argument for keeping a second, independent voltage reading even after you own a shunt. Not because voltage is good — the battery guide is fairly rude about voltage — but because it fails differently, and two readings that disagree tell you something one confident reading never will.

The combinations

A crossed threshold is a question. These are the second readings that answer it, in the shape the cookbook uses throughout.

• Crossed the floor, with a modest discharge current — you simply used the energy. Go and charge it.
• Crossed the floor, with a strongly negative current — something is on that should not be. Find it before you start the generator, or you will be doing this again tomorrow night.
• Bank falling and the pedestal still reads 120 volts — the converter has stopped charging, which is its own article. Shore power is not the problem.
• Bank falling and the pedestal is dark — the outage is the problem and the battery is doing exactly its job. Start there instead.
• Percentage at 25 while voltage still reads 13.1 on lithium — trust the percentage. That is the plateau, and the voltage was never going to warn you.
• Percentage at 60 while voltage reads 12.5 on lithium — trust the voltage. The count has drifted; charge to full and let the shunt resynchronise.
• Hub went offline, and its last voltage was already low — the bank ran out and took the hub with it. If the last reading was healthy, it was connectivity, not the battery.

What none of this tells you

How old the bank is, or how much of its capacity is left after five years of use. A shunt counts against the capacity you configured, not against the capacity the battery still has, so an ageing bank quietly reports a percentage of a number that is no longer true. Nothing in the catalog measures that, and it is one of the reasons the hours calculation should be treated as an estimate rather than a promise.

And the free alert everybody gets — the hub going quiet because the bank it runs on died — is the last message rather than a warning. It should be assumed, never relied on.

What to do about it

If you boondock or run lithium, the shunt’s state of charge is the primary rule, because it is the only reading here that measures rather than infers. Add a second voltage source as the cross-check, and on lithium set its critical rule right at the cliff as the backstop for a percentage that has drifted. A shore-powered or stored rig can stop at a cheap voltage monitor and be well served.

Then move both thresholds up until the alert would reach you at a moment when you could still do something about it. That is a different question from where the battery gets unhappy, and it is the one worth answering. The floor that fires at two in the morning and reaches you at seven has to be set for seven.

Where to get one

The shunt and the cross-check, in that order. Nothing comes off a Victron until its advertisement key is in the hub, so budget two minutes for that on the day it arrives. The compatibility database lists everything else that reports a house-bank voltage.