Using a battery Charger and Solar Regulator in Parallel

I went down to the boat today to find I left the house bank on. Yep - very flat house battery. Probably an expensive mistake.

Anyway, my setup has both a 12v solar charge controller (instruction manual here) which has always maintained the battery nicely and a 240v Dolphin - series 1- battery charger (instruction Manual here) which I have never had to use since installing the solar panel. That is until today.

Anyway in the wash up I plugged in the battery charger and normally with a flat battery it would go straight into a boost charge and I knew that because the charger fan would start running. But not today. After taking a few measurements I decided that the problem was with how I had wired the two in parallel.

It would seem that because the battery charger sees quite a high voltage coming from the solar panel it thinks the battery has a high voltage and goes into float charge mode (with relatively low current input). Equally the solar controller is only putting in a low amperage because it is not that sunny outside.

So my question is how should this be done? Is there a better way without spending a fortune? Should I just isolate the two inputs with a switch? Can I use them in series?

What's your thoughts everyone?

 

Cover the solar panel with a towel.

 

That's exactly what I did today...
I guess my question is there a better way to do the installation.

 

A switch would be the easiest. You can use a double throw switch where you choose charger or solar panel.

 

There is nothing wrong with your setup!
With a flat battery and no bright sunlight the battery voltage will remain low so your charger should go into boost mode. If it doesn't there must be something wrong with the battery bank or - more likely - the wiring. One corroded spot is enough to let the charger make a wrong decision.

 

For most modern controllers and chargers, they won't connect to a flat battery at all. They need about 50% voltage minimum in order to run their diagnostics and auto range. A charger that's looking for a battery and seeing a solar panel my not know what to make of that. These things send out a burst of current and look for a result that is tied to the sensed voltage. The response of the battery that is flat will not be what the charger expects if it is reading the solar voltage.

The trend is definitely towards having a single inverter/charger device that manages control of power distribution among all sources and loads, of which the battery may be either. There are several with slots for MPPT solar controller modules, so you can size the array independently of other components. They will manage the batteries and control generator starts. These are off-grid devices.

For on grid devices, there isn't much of a market for what you are trying to do. There is something called AC mini-grid coupling that lets a grid-tie and off grid system cooperate so that the capacity of the grid-tie device is available for off-grid use if the grid is down. But I don't know of any small, conventional charger that will combine with solar in a sensible way if the solar has any significant capacity compared to the AC charger.

The towel sounds pretty good to me.

 

A modern charger will not go into any charging mode if the battery is below a certain threshold. The normal procedure is to generate small bursts to see if the connected device really is a battery. That is not completely fail-safe, if you connect a large capacitor it will try to charge that as soon as the minimum voltage level is used.
The charger cannot see the solar voltage, it is connected to the battery only. A really flat battery will have a low enough voltage for the charger to start doing its job.

I use a charger that only comes in when the battery voltage drops below 11.5V for several minutes, a situation that only occurs after subsequent days with heavy overcast. The charger keeps working until approx. 14V is reached, then returns to sleep mode.