Xantrex Truecharge 20+ fuses

[David W]

I installed a Xantrex Truecharge 20+ on the weekend. I have a Trojan group 24 deep cycle and a starting battery.

I used #10 wire for the DC wiring and #14 for the AC as per the manual. I fused the positive DC circuits with two 25A fuses (one for each battery) and the AC with a 15A breaker as per the manual. Settings set to warm temperature, flooded batteries and three stage charge.

Plugged in the charger and all seemed exactly normal, just as it was when I connected it at home using the same configuration.

We went sailing yesterday and when I returned to the dock and plugged in my shore power again, I checked the lights & fuses and noticed that one of the 25A DC fuses was blown. The fuse on the house battery was fine.

What could have caused this to occur?

The only thing I can think of is that I started the engine with the battery switch in the "Starting" position and the shorepower was still connected, therefore the charger suddenly had to produce more than 25A. How it might have done this I am not sure.

Or did I backfeed more than 25A when the alternator started putting out its rated 35A, so the fuse blew to protect the charger?



Thanks!
David

[David W]

could be anything...

My notion that an excess of current from the alternator blew the fuse was shot down because the tech said the current from the alternator will seek the path of least resistance which is the battery and not the charger.

His only suggestion was that perhaps the fuse was not a 25A but something less in a package of 25A fuses...

Seems very odd to me that the starting circuit was the one that blew the fuse. The alternator is connected to both batteries so why would only one blow?

[Alan Wheeler]

Hi David, so do I understand this part right. You started the engine with the shorepower still connected?? If yes, then that is more than likely the culprit. The draw from the Starter may have been more than the battery could supply. Even just for a split second. The initial get up and go of a starter can draw horrendouse amounts of current and ontop of that current draw, the battery takes a little time to generate it's full current output. The charger would be supplying a high current to that starting circuit for a time till the battery could supply enough and the starter was not drawing so much. It could also be possible that the starter draws more than the battery can deliver anyway. So in the end, the charger is supplying max current into a very low resistance starting circuit and the whole circuit will look like a short to the charger. Hence the fuse would fail.

[David W]

Because had only my #1 battery connected and only the #1 battery fuse blew that is the only logocal explanation.

While we were sailing I tried starting the engine again from the #1 and found that the battery was too discharged to spin the engine. So that leads me to believe that yes in fact the #1 battery was unable to supply sufficient current to start the engine @ the dock causing the charger to supply its maximum (obviously more than 25A) current and blew the fuse on the #1 battery charger circuit.

Off to the battery store for a new starting battery... it's ok, it is of unknown vintage...

Thanks Alan!

David

[Rick]

Here's the deal with modern chargers. You should be able to start your engine with or without the charger on, with or without the shore power on, with or without any other charge source attached. No fuse should blow.

You probably do not need a fuse anyway in the charger output. Regardless, what happened (I believe) is that the charge on the capacitor internal to the charger provided sufficient current (far beyond the continuous charger current rating) to blow the fuse when the load (starter motor) caused the voltage to sag (probably below 8 Volts).

If you use a slow-blow fuse rated for motors (they have many seconds of rated heating before blowing if you find the correct one) you will not have nuisance fuse blowing. The problem is that such rated fuses are, in general, much more expensive and much larger than the usual fuses which are relatively fast blowing and might not be available in the same package for the same current rating.

Please note that the Trucharge chargers are ALREADY overload protected, so why fuse the output? Yeah, I know, "belt and suspenders". Such philisophy is not necessarily good on a boat which must be reliable without built-in excessive overprotection which can DECREASE overall operational reliability as you just experienced.

Regards,
Rick