Solar panel 200watt - Not what I expected

[Chilly]

Hi, looking for a bit of help, if you can it would be very much appreciated.
Just installed a Renogy 200watt flexible panel on my van roof, I have connected it to a Renogy 20amp mppt controller, then to an AGM 130ah battery.
Not sure if it’s a problem or not but I have searched and can’t seem to find anything. Mostly sunny day yesterday with a bit of haze, was getting 34watts from the panel, yet when I started the van up the solar wattage went up to over 160watts, then when I turned it off the solar watts went down again. Is this normal, do I have a problem, what is going on…?

 

[Chilly]

I’ve attached two pics, the low reading is just solar, the higher reading is with engine running

 

[Bigsidavies]

When you had the engine running was there something connected to the leisure battery causing a draw from it?
The actual starting of the engine could be a coincidence.
In both pictures the battery is showing as being 100%, so no matter how big your panel is it will be putting little to no juice into it.
The first pic shows ‘boost’ whatever mode that is from your mppt charger, but the second shows ‘bulk’.
That makes me think that at the time of the second pic there is something that is drawing maybe 6amps that has kicked your mppt into bulk and therefore pulling 6 amps from the panel.
But I’m far from an expert on these things.

 

[Chilly]

Hi, thanks for relying, I’m new to this, I hadn’t even picked up on the Boost/Bulk thing, just been reading up on it and there is a float mode it goes through. So boost is when it’s charging the battery, bulk is when it’s nearly charged and float is when it’s trickle charging. Not sure what the draw was unless the mppt picks up the charge from the alternator via the leisure battery…? I am going to put a multimeter on the panel tomorrow to see what I’m getting, I hope I didn’t damage it when pressing down on it when making it adhere to the sealant…..

 

[Bigsidavies]

It shouldn’t be affected by any dc-dc charger that’s powered by the alternator.
Does it have an upgraded stereo with a power amp or powered sub that could be running off the Leisure Battery?
Might be an idea to turn the key just to ignition on and see if there is any effect on the charge rate.

 

[roadtripper]

With most battery chemistry bulk charging will roll off somewhere around 80-85% so if you want to confirm how much your system can charge in real world conditions you'll need to put a load on your leisure system and get it discharged down to the 60-70% region first.

Otherwise you're testing the speed of a petrol station pump with a full tank - it'll just keep stopping because it's full...

 

[roadtripper]

How are you DC charging your leisure battery?

If you have a basic split charge relay but start/stop then that might explain the behaviour you see.

With start/stop your van will only charge the battery to around 80% to leave room for regenerative braking. A split charge relay just makes the starter and leisure battery into a bank so it will be charged the same.

Then when you stop and the split charge relay drops out the starter will stay at 80% but your leisure can now get to 100% with the solar.

Then when you next start the van the split charge relay will connect an 80% battery to a 100% battery as a bank - and charge is going to flow pretty quickly from the 100% to the 80% until they are equal charge. At which point your solar controller is going to see around a 90% charged battery and ramp up the charge current (if it has enough sun)

 

[Chilly]

Thank you all, it is just a standard split charger, nothing is drawing on the leisure battery as know. I have just put a multimeter directly on the panel plugs. The stated panel VOC is 23.9, testing it I got a reading of 23.4. The stated panel IOC is 10.74, I got a reading of 4 ish as it very cloudy

 

[roadtripper]

Then what you are seeing is what I would expect, with the ignition on the van will not charge beyond around 80% but the solar charger will be trying to get the bank to 100%.

With a split charge relay there is no separation between the batteries, when the ignition is on your van electrics and leisure electrics are connected and all running off both batteries as a battery bank, you can't treat them in isolation at that point.

If you want to have 2 wholly separate electrical systems that's what a DC-DC charger gives you.

 

[Chilly]

Then what you are seeing is what I would expect, with the ignition on the van will not charge beyond around 80% but the solar charger will be trying to get the bank to 100%.

With a split charge relay there is no separation between the batteries, when the ignition is on your van electrics and leisure electrics are connected and all running off both batteries as a battery bank, you can't treat them in isolation at that point.

If you want to have 2 wholly separate electrical systems that's what a DC-DC charger gives you.

Well explained thank you. Will that make any difference with the ignition off, and the panel is only reading say 27 watts on a sunny day? I might be missing something here, thanks for your help.

 

[roadtripper]

Well explained thank you. Will that make any difference with the ignition off, and the panel is only reading say 27 watts on a sunny day? I might be missing something here, thanks for your help.

With ignition off the two systems will then be separate, but the battery will likely be at around 80% at the point you park up. It will then charge further from the solar.

You solar will charge the combined battery bank with ignition on, but it won't make much difference to the state of charge at the point you park up as the more energy the solar puts in the less work the smart alternator will do.

This is the downside of the basic split charge systems, any "extra" you put into the leisure battery from solar or EHU above 80% while parked up will be lost within in a few minutes of ignition on to the van electrics. It's not really lost as you will have marginally better fuel economy from the alternator working less hard, but if you wanted to keep it for camping later that evening it's lost for that purpose.

It's only really an issue if you are touring and moving every day, or needing to put the ignition on regularly. If you're parked up for a week of summer camping it's much less of an issue.