Two solar chargers off one panel

[TomS_]

Hi all. I recently setup a solar panel on top of my garage because I can't get mains to it. Off the panel I have two chargers: one charges an old car battery which I use to power some lights, and the other I hook up to my motorbike when I'm not using it, just to keep the battery topped up.

I noticed that when measuring the voltage coming from the panel that I only see roughly the voltage of a battery, 12.something volts. Open circuit it gets up to somewhere like 20 or so volts.

I was half expecting to see a higher voltage from the panel, because I believe that you need something upwards of 13-14 volts to "properly" charge a SLA battery? I am expecting this because I would expect the charger to step that voltage down to the required charging voltage on its battery terminals.

I just realised that because the chargers are common positive that what I am seeing probably *is* the battery voltage itself.

So I'm wondering, is having two chargers connected in parallel to the same solar panel a good idea? What kinds of problems am I setting myself up for by doing this, if any?

If that's the case, how would I go about measuring the voltage from the panel while a load is connected, or do I just have all of this backwards?

Thanks!

[NiHaoMike]

MPPT controllers probably will not work correctly, while PWM controllers will work just fine. But rather than a second charge controller, what about just connect the batteries in parallel with a PTC to limit the maximum current?

[TomS_]

I do believe they are PWM based. At least, if you look up this model on somewhere like Amazon, they all claim to have some special "PWM technology".

I guess the reason why I dont want to hook the batteries in parallel is the chargers also draw from the battery to supply a load output (and power themselves for monitoring). Ive actually built a little relay timer/sequencer to disconnect the motorbike battery from its charger outside of daylight hours so I can leave it connected without worrying about that charger draining the battery.

I dont mind that for the battery that powers the lights, but I want my bike to start in the morning. 

I could probably use some better chargers, but these were given to me by a friend who had them laying around, so Im going to run with them for the time being.

[grifftech]

how about this

[TomS_]

Would the diode prevent the bike battery from getting the proper charging voltage?

[grifftech]

it would but less than .5 volts for a Power Schottky rectifier https://www.st.com/resource/en/datasheet/stps2045c.pdf

[daveatol]

MPPT controllers probably will not work correctly, while PWM controllers will work just fine.

I believe PWM controllers connect the panel straight across the battery for most of the charging time, and only start pulsing on & off (i.e. the PWM) towards the end of charge. This will pull the panel voltage down to that of the battery.
I'd expect two MPPT controllers would work, as they'd both adjust to get the most that they could from whatever voltage the panel is dragged down to.

Alternately, I would think a CV/CC DC/DC converter to provide a float voltage to the bike battery from your large battery would work too. Then remove the other PWM controller.