Interpreting charge and voltage graphs

The Battery Recorder graphs allow you to see at a glance how quickly your batteries are charging or discharging.

Here’s graph, showing a battery’s charge over 24 hours

It’s an AGM battery, charged from a solar panel, and is running a fridge – a fairly typical camping setup. It had been a cloudy day, with not much sunshine after midday.

The sharp dips happen when the fridge’s compressor runs. You can see the fridge running every hour or so in the early afternoon, and then less often through the night – it doesn’t have to work as hard when the outside temperature falls.

The dips are caused by the battery’s internal resistance. Whenever a battery or solar panel supplies current, its voltage falls while the current is flowing. But it’s only a temporary effect – remove the load and the voltage recovers. This is true of all batteries, but especially so for lead-acid batteries, including AGM.

An AGM battery’s voltage can give a good estimate of its state of charge, but only when the battery is at rest (not loaded). See Estimating state of charge from battery voltage for more details.

That means that when you see dips like this in a charge or voltage graph, you should ignore them and instead look at the level between the dips to get a true picture of the battery’s performance.

Let’s look at this graph again, piece by piece, to see what’s really going on.

At first, the sun is shining and our battery is 100% charged.

The occasional dip is due to the fridge running briefly, every hour or so.

In between those dips, the charge estimate sits at 100% and we can say that the battery is keeping its charge at 100% during this time.

Now the sun has gone, but the air is still warm, so the fridge continues to run, every hour at first but slowing by nightfall.

Each time the fridge’s compressor runs, the battery voltage dips and recovers, but not as high as before.

With the sun not shining, the battery is losing charge whenever the fridge runs, as we’d expect.

It drops from 100% at 2pm down to around 90% by 7pm, a loss of 10% in 5 hours. At this rate it will be down to 50% (the lowest that AGM batteries should be allowed to go) in another 20 hours. So we should be ok until around 3pm the next day, if nothing changes.

In the cool of the night the fridge doesn’t have to run as often, so it’s using less power. The battery charge is now dropping more slowly, from around 90% at 7pm to 85% by morning.

At this rate the battery would easily last a couple of days – except of course that as the day gets warmer the fridge will draw more power, and the battery will lose charge more quickly again.

Unless of course the sun comes up…

Just after 5.30am, the first glimmerings of sun appeared, and the battery started charging.

By 6.30am it was back to 100%!

Then the fridge came on again, and there’s another big dip. But remember that this is only internal resistance at work. The charge didn’t really drop as much as it appears to.

With the solar panels supplying power, the battery is quickly back to 100% and stays there.

Overall, from this graph we can conclude:

  • The battery keeps its charge while the sun is shining, even when the fridge is running.
  • The fridge runs more often, drawing more power, when it’s warm outside.
  • Overnight we lost around 15% of the battery charge.
  • This tells us that we could go at least a full day without charging.
  • The battery charges really quickly when the sun comes up – so maybe we have a bit more solar capacity than we really need.

And that’s what it’s all about. How well is the battery holding up, and does it charge fast enough?

The answers from the graph are clear – our battery and solar setup is performing well.