Start battery maintenance

[Rwnielsen]

Good morning all
This is a simple question, for the dedicated start batteries. How many watts of solar for a couple of group 31 batteries?
Just a dedicated solar panel with a battery tender. Amazon sells units anywhere from 10 watts to whatever which, realistically is about 35-40% of that.
Thanks for your help
Rick

[dzl_]

I suppose a start to answering this question would be finding the datasheet for your batteries (or barring that, general information on the battery type) and seeing what the 'self discharge rate' or similar spec is.


I've usually seen this indicated as %, then if you know the Watt-hours (or amp-hours) of your battery, you could determine roughly how many Watt-hours you need to supply to keep your batteries topped off.

[john61ct]

If the battery is healthy (a big if)

and 100% Full when going into standby / storage mode

and in a location / season with decent insolation factors (also highly variable)

then 20W is plenty to counteract self-discharge with the battery **completely isolated**.

However, if there are parasitic loads that need to continue to be serviced, e.g. ECU, security / safety systems, keeping stuff warm?

then you need to measure the Ah per week consumed and add that as well.

[bubb, the real one]

As they said, it takes 5-10 watts per battery to prevent parasitic discharge depending upon batt size, 20 watts should work, to be on the safe side I would check batt voltage weekly for a few months to confirm

[Rwnielsen]

That's good enough for me, thanks guys for the advice
Rick