Help me understand my solar charging

[Little Dorrit]

I have a 30W solar panel which is connected to a controller which has two charging outputs for two separate isolated batteries. It's a 12/24 Volt charger with a maximum 10A output. There is a 70 Ah starter battery and a 100 Ah leisure battery.

The two batteries are also connected via a shunt to a Philippi Battery Monitor which gives the Voltage and Amp usage of the batteries. If I charge the batteries using my Sterling Pro Charger I get a reading showing positive charge of about 6 - 7 Amps (when in absorption stage) going into the batteries and a voltage of about 14.4 (while charging). When disconnected the battery voltages drop. In both cases I have checked the Philippi voltages using a voltmete and they appear to be accurate. So the battery monitor appears to be giving me an accurate picture of what's going on.

The controller (PWM) charges battery 1 (leisure) up to 90% and then battery 2 up to 90% and then alternates charging both in turn. Green flashing lights indicate the state of charging - all seems to be as it should.
I have checked the voltage from the solar panels and I am getting about 20 Volts, my understanding is that a 30W solar panel should output 18 - 20 Volts (about 1.7 A) so again all is good here.

I am guessing about 30% efficiency would not be unreasonable at this time of year when the solar panels are in full view of the sun.

So taking all the above into account what should a reasonable charging current and voltage be when attached to the 100 Ah leisure battery and how would I check this?

I'm not expecting miracles but as things stand my battery monitor (and voltmeter) is showing a voltage of my starter battery at over 14 Volts when connected to the solar panels but my leisure batteries show about 13.2 Volts when it's got 92 Ah of charge with just a trickle of amps 0.05. Are these voltage discrepancies normal or are they 'confusing my controller'? What chargng current should I observe?

FYI I have checked continuity of all cables and the cables sizes are well within specified limits for the short runs I have of just a few metres.

 

[lpdsn]

Unless you've got an MPPT controller you're not going to gain any current at the output of the controller. So say you're getting something like 1.2A out of the panel at 20V, the max the controller will put into the batteries is something getting on for 1.2A.

Next it depends upon the output voltage of the controller. It might be set to 14.4V but many are programmable, even the cheap ones, so it might be something different. It is then driving against the 13.2V of the battery and the batteries internal resistance, so there's a relatively small voltage actually driving the charging current through your battery.

You don't say whether you're taking these meaasurements with any load on the domestic battery. Best turn everything off (and get rid of any always on load) for accurate measurements.

Do you get variations in the charging current when the sun comes out or goes behind a cloud?

 

[Little Dorrit]

Unless you've got an MPPT controller you're not going to gain any current at the output of the controller. So say you're getting something like 1.2A out of the panel at 20V, the max the controller will put into the batteries is something getting on for 1.2A.

Next it depends upon the output voltage of the controller. It might be set to 14.4V but many are programmable, even the cheap ones, so it might be something different. It is then driving against the 13.2V of the battery and the batteries internal resistance, so there's a relatively small voltage actually driving the charging current through your battery.

You don't say whether you're taking these meaasurements with any load on the domestic battery. Best turn everything off (and get rid of any always on load) for accurate measurements.

Do you get variations in the charging current when the sun comes out or goes behind a cloud?

Thanks for the response. It's a cheap controller so I'm guessing that it may be worth investing ina MPPT controller although with only one panel I was hoping the PWM controller would be ok; perhaps it's a very basic one and not up to the job?? I'm measuring with no loads on the batteries so have no reason to question the readings are an accurate reflection of whats really going on with the batteries. I just find it odd that one battery appears to see a higher voltage 14.2 than the other one. I'm going to go down today and will do some more checks as it's quite a nice day the poanels should be working well and I'll try covering them up and seeing what difference it makes plus I'll swap the charging leads over too to see if somethings wrong there.

 

[lpdsn]

An MPPT controller will cost you getting on for as much as a new 100W panel, so not a worthwhile investment for a 30W panel.

I'm not sure there's anything really wrong with your set up. You have a 30W panel trickle charging two batteries and it sounds like it is doing that. I suspect the smaller battery has reached a higher voltage because firstly it is smaller and secondly it probably had no discharge beyond self-discharge when you started charging.

Your best bet is probably a bigger (or extra) panel if you have somewhere to fit it. Panels are getting cheaper all the time and the basic inflexible ones are now about £1/W, with the semi-flexible ones about 30% more.

 

[Little Dorrit]

An MPPT controller will cost you getting on for as much as a new 100W panel, so not a worthwhile investment for a 30W panel.

I'm not sure there's anything really wrong with your set up. You have a 30W panel trickle charging two batteries and it sounds like it is doing that. I suspect the smaller battery has reached a higher voltage because firstly it is smaller and secondly it probably had no discharge beyond self-discharge when you started charging.

Your best bet is probably a bigger (or extra) panel if you have somewhere to fit it. Panels are getting cheaper all the time and the basic inflexible ones are now about £1/W, with the semi-flexible ones about 30% more.

Thanks for your considered response. There are now MPPT controllers available for less than £30 not sure on the quality but there are a few available on eBay from China.

 

[lpdsn]

Thanks for your considered response. There are now MPPT controllers available for less than £30 not sure on the quality but there are a few available on eBay from China.

There's a few guys on YouTube that take these things apart to show the internals so worth checking for those videos before buying one. There are controllers that are mislabelled as MPPT.

 

[duncan99210]

You're asking quite a lot from a 30 watt panel in the UK. Rather than getting a new controller I'd follow lpdsn's advice and fit more panels if you think you need them. An additional 30 watts of panel will double the input, will easily be dealt with by the existing controller and not cost much more than a new controller when I don't think that there's anything wrong with your existing setup.

 

[Little Dorrit]

You're asking quite a lot from a 30 watt panel in the UK. Rather than getting a new controller I'd follow lpdsn's advice and fit more panels if you think you need them. An additional 30 watts of panel will double the input, will easily be dealt with by the existing controller and not cost much more than a new controller when I don't think that there's anything wrong with your existing setup.

I'm getting 0.03 Amp while 100 Ah battery is showing 94 Ah and 13.26 Volts. Should I not expect more on a cool sunny day in May?

 

[BabaYaga]

I'm getting 0.03 Amp while 100 Ah battery is showing 94 Ah and 13.26 Volts. Should I not expect more on a cool sunny day in May?

Could it be the case that the battery is already practically fully charged (monitor misreading a bit) and the controller has gone into float mode?

 

[PaulRainbow]

An MPPT controller will cost you getting on for as much as a new 100W panel, so not a worthwhile investment for a 30W panel.

Absolutely right, be a waste of money.