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Hi. So when my mppt 75/15 is sending 15.5 volts to my battery. Which as now happened on 3 occasions. Why hasn't 19amp fuse blown on the cable going to the battery. I can only think the last time it happened. The chatge controller was in the garage. Once it warmed up it worked as normal and went to float after a while. And didn't go above 14.2 on volt meter
Sorry 10amp
Sorry should read 10amp fuse
Hi @Steve, a couple of things: First, the fuse between the 75/15 controller and battery should generally be a 25A fuse, since the controller could put out up to 15A and you don't want your fuse blowing if the controller reaches its full potential. This depends on the wire gauge and length you have running from your controller to your batteries, however, so maybe you've sized the 10A fuse according to your wire gauge - in which case, leave it as it is.
Second, the fuse you have didn't blow because your high voltage does not equal high current; the fuse is a 10A fuse, which means it will blow if more than 10A flows through it for a certain period of time, but the fuse can probably handle 32-48v or more. A little 15v running through it isn't something the fuse is going to notice.
And, finally, you're most likely seeing the voltage that you are as a result of the controller compensating for low temperatures, which so long as you're charging a lead battery, is perfectly appropriate. Pb batteries require higher voltage to charge at low temperatures because their internal resistance is higher; then as the battery/controller warm up, the voltage will drop off. This behavior is addressed in section 1.6 of the user manual.
It came with 15amp fuse but after seeing over 15v going to battery. I put 10amp in hoping fuse would blow before overcharging
@Steve, volts do not equal amps... the fuse won't blow due to overvoltage, it only protects from overcurrent (amps).
15v going to a gel battery may well be overcharging - this depends on the manufacturer recommendations for your particular battery. If needed, you can dial down the Bulk/absorb voltage in the controller so that when it's temp compensating it doesn't go over your battery recommendations... it all depends on the manufacturer recommendations.
As an example, Victron's gel batteries should have temperature compensation applied, as per their user manual:
"The charge voltage should be reduced with increased temperature. Temperature compensation is required when the temperature of the battery is expected to be less than 10°C / 50°F or more than 30°C / 85°F during long periods of time. The recommended temperature compensation for Victron VRLA batteries is -4 mV / Cell (-24 mV /°C for a 12V battery). The centre point for temperature compensation is 25°C / 70°F."
