On Tue, May 09, 2017 at 02:17:36PM +0100, Phil Thane wrote:

On the other hand it would be good to have the solar panel keep the engine battery topped up when we don't use it for a while, so will investigate split charging in both directions. Might get complicated unless someone makes a specialist control box for just that scenario, in which case it'll get expensive!

There are *many* charge controllers for solar panels. A standard solar panel (as in a house type) puts out around 30 to 35 volts and maximum power per panel is about 260 watts. I don't know if you have anything this big or not, but anyway....

To treat your battery nicely (and thus extend its life as far as possible) you want to keep it as near fully charged as you can all of the time. It's discharge/recharge cycles that kill lead acid batteries and especially deep discharging. Car/starter batteries are designed for short bursts of high current (i.e. starting cars) and are particularly bad at coping with deep discharge, however even batteries designed for it ('leisure' or 'deep cycle') are worn out the same way basically.

So, the basic charging cycle for a lead-acid battery is controlled by the battery's voltage:-

Up to about 13 volts charge as fast as you can (not more than about 25% of the 1hr rate though, so for a 100Ah battery about 25 amps is a good maximum).

Then tail off the current as the battery reaches full charge, the 'fully charged' voltage is somwwhere around 14.5 volts but depends on temperature and battery type.

Having reached 'fully charged' back off the charging voltage to 13.5 volts which maintains the battery's charge but doesn't make it gas and lose electrolyte. This is called 'float'.

The above is pretty simplified, many modern chargers do further clever things to improve the speed and battery life.

Thus a good battery charger does need to be quite clever, the types for running from solar cells (in improving 'goodness') might be:-

Simple series regulator, takes current from panels and simply drops it resistively to the voltage needed to charge the battery. Very inefficient as you are losing around 2/3 of the power coming from the solar panels (if they are outputting 35 volts).

'Switching mode' regulator, or something like it. This has a switch mode PSU of some sort in it and thereby uses more of the power from the panels.

MPPT regulator, this is an extension of the switching mode one and actually maximises the power drawn from the solar panels by keeping the current (and thus voltage) drawn from the panels at the optimum level. It then converts (using a switching regulator) to the voltage required for the battery.

As in all things electronic there's a huge range to choose from (especially direct from China) and a mix of good, bad and awful.

I have an MPPT regulator that actually has two outputs, one main output aimed at the leisure battery and an auxiliary output for keeping the starter battery charged. These are fairly expensive though, made by Votronic in Germany, their web site it quite helpful and interesting. If you really want to see silly prices then look at Victron stuff!