charge controller readings

**cargillfarm** · October 17th, 2011, 20:41 PDT

I have a trace C40 controller that I have connected to 6 siemens 17v 75 watt panels. The panels are wired series parallel. This feeds into a 24v 340 amp hour battery bank. I have just set this up. I thought that the panels would dictate the current being fed to the batteries but it seems to be the other way around i.e. the batteries seem to be restricting the panel output. Yesterday was a very sunny day and the C40 progressed from about 4 amps in the morning to 11 amps at midday but the watts always progreesed at a ratio so that the voltage stayed just over 24v. eg 11amps/ 268 watts = 24.3 volts. The reading at the panels is about 2v higher than at the C40. My multimeter won't measure over 10 amps so I didn't check the amps at the panel. When I disconnected the C40 from the batteries the voltage stayed much the same. How do I get the voltage up so that it charges the batteries?

**Cariboocoot** · October 17th, 2011, 21:03 PDT

Let's see. Six 75 Watt panels is 450 Watts. Less a typical 77% efficiency derating is 346 Watts. Divide by 24 Volts nominal = 14 Amps peak current, which is 4% of 340 Amp hours. You're seeing 11 Amps, which is only 3%. You're going to run out of sun before that array manages to get the battery Voltage up to Absorb level.

Solution: get enough panels to charge the battery bank. You've got about 1/2 the amount needed.
Before you do that, find another way to charge the batteries before it's too late.

Just to cover bases, check the Isc on each panel and the insolation too. Watch out for long wire runs from array to controller or too small wire size; either can eat up Voltage when under load.

Oh, and you shouldn't disconnect a controller from the batteries if there are panels connected to the input. It can be bad for the controller.

**cargillfarm** · October 17th, 2011, 21:14 PDT

I have a generator as well which we have been using for several months to keep the batteries up. If I get the batteries up to absorb with the generator then will the panels finish them off?

**Cariboocoot** · October 17th, 2011, 23:11 PDT

Originally Posted by cargillfarm

I have a generator as well which we have been using for several months to keep the batteries up. If I get the batteries up to absorb with the generator then will the panels finish them off?

They should, providing that 17 Volt panel spec is the Vmp and not the Voc.
When you're reading 11 Amps output on the charge controller check the Voltage coming in to it from the panels. Hopefully it will be around 34 Volts.