The meters with current shunt proved to be inaccurate because the supply circuit and the measuring circuits are not isolated. I bought some B0505S 1W 5V DC to DC converters, which allowed the 5V supply to float. Now the midday sun in SE England in May was giving me a reading of 4A into the battery, whereas before it was about half that.
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Leisure Battery Charging Circuit
Without doubt batteries and charging causes the greatest amount of confusion on the online forums. This is not helped by the suppliers intent in making users part with as much money as possible.
Even tring to ascertain if a battery is actually deep cycle is very difficult.
One thing I do believe though is that flooded lead acid is the simplest and most robust tech. As long as the battery is not seriously discharged, and is maintained correctly, then they should last a long time. Multi-stage charging is key to this, but the costs are high.
My approach is to use a solar panel with mppt controller, coupled with alternator charging when traveling (if required). In addition I have a ctek charger permanently connected, so I can topup if on hook up.
There are three states
1) Engine off - Solar charges Leisure battery & any spare goes to the Starter Battery. The leisure battery supplies the fridge etc.
2) Engine running - Solar still charges Leisure battery but the alternator supplies the fridge etc.
3) Engine running - Alternator supplies the fridge etc. and also charges the leisure battery. This is selected via the 2nd switched relay.
I way lucky with the wiring on the Expert, in that there is an "engine running" relay that I was able to pick up using a piggy back fuse connector. This saves having to draw current from the alternator sense wire - which can cause unexpected results.
Relay block with fuses
Finished controller, in the passenger footwell, with the relay unit sitting above under the glovebox
Overall the design should provide controlled charging via solar along with the chance to optionally boost the charging via the alternator.
To monitor what is going on, I fitted a couple of current measuring shunts - one in the leisure battery return, and one in the Fridge and Lights return so I can get some idea of the current in and out of the battery.
These are cheapo eBay meters from the far east. One problem that I did think of, is that these meters do not show reverse current - so the top one shows the current into the battery, and the bottom one show the current consumed by the fridge etc. They have separate connections for the meter supply, but I was getting very odd results, until I disconnected the ground wires. They are not ideal, but for £10, they will do for now
I'm started using very conservative fuses - 15A between the starter & leisure battery and 40A relays.
However I ran the fridge for a while to run the battery down to 50%, and checked the current flow when the alternator kicked in. I was a very surprised to see this initial inrush was almost 30A. It did reduce after a few moments, but it was way higher than I expected. I do wonder what current would flow if the battery was severely depleted, and what it would do to my 40A relay!
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