[ALUG] lappy aboard

Chris Green cl at isbd.net
Tue May 9 15:05:14 BST 2017

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

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!

Chris Green

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