Battery Charger Lessons

[Mike and Theresa Vaccaro]

We recently completed an extensive upgrade to our electrical system, including a new electrical control panel and breadboard as well as addition of a Link 10 monitor and new wiring, fuses, etc.  Only one week later, the house bank charger died!

Turns out, it was really our fault; and since there's been quite a bit written about chargers, this is just a lesson learned:  The charger has to be properly sized to support your system(s), since it will also function as a DC power supply when the boat is running on shorepower.

What caused the charger to fail after only 13 months was the large house bank (440 amp hours) made up of 4 golf cart batteries in conjunction with demands on the DC system when dockside.  The charger we had installed was simply max'd out and failed trying to recharge the house bank and running DC systems at the dock simultaneously.

A simple rule of thumb is that the charger should be capable of delivering 6.6% of the total bank capacity in amps.  Thus, for a 440 amp bank, a charger should be capable of delivering at least 29 amps.  

Our "dead" charger was a 10/10 type bridged to provide 20 amps.  The new charger is a Guest three stage charger capable of providing 30 Amps to the house bank and 10 to the starting battery.  To size the charger, I provided an energy budget to the folks at Guest who were very helpful.  They indicated that if a charger is properly sized, and connected to a reliable dockside power system, it shouldn't fail in short order!  If it does, odds are it's improperly sized for the installation.

So, although it's a good idea to maximize battery bank size from a BATTERY MAINTENANCE standpoint, bank size might be limited by CHARGER CAPABILITY--unless you buy a very expensive high-capacity charger.

The charger we have ordered as a replacement is a Guest model 2634, waterproof heavy duty system.  It is only available by special order, but is resonably priced.  This charger can tolerate modified sine wave power input from our engine driven generator, and is also capable of dealing with our lead acid house batteries and AGM starting battery simultaneously (which systems like the Truecharge are not).  It also replaces the two separate chargers we were using (one for the house bank and one for the starting battery).

One more thing for the cross-check!

Cheers,

Mike

[Mike and Theresa Vaccaro]

Post-mortem update:  What killed the battery charger?  Heat.  

The buildup of internal heat resulted when the battery charger output was "bridged."  This means that the two 10 amp outputs were connected to the same battery bank for a total output of 20 amps.  This is important since bridging output is an accepted technique for increasing amperage.

For the first year the charger was installed, only a single 10 amp output was used.  Although our housebank is rather large, it turns out that this was adequate as the boat was consistantly hooked up to shore-power when not sailing (which is most of the time).  Although insufficient to meet DC needs at the dock, the batteries more than made up for this.  The bottom line is that the 10 amp circuit was maxed out at all times other than "float" mode, but since only half of the charger was working, it was able to cope with the heat generated.

When we bridged the output, we essentially created a 20 amp "max'd out" charger, but now the system was generating twice as much heat as before.  It only took several of these heat cycles to cause failure.  

Lessons learned:  

1.  If you have an "undersized" charger, talk to the folks that made it and do a reasonable energy budget before you you bridge the output in an attempt to "improve efficiency."

2.  If the budget allows, buy a properly sized charger or chargers for your system.  Don't forget to consider the charger when you upgrade your electrical system or expand your battery bank(s).

Cheers,

Mike

[Stu Jackson]

Mike

Nice analysis and interesting information.

One question:  what on earth (or on the boat!) were you running that required over 20 (or even 10) amps?  The fridge is 5 to 6, and say a few lights make ten to fifteen. So the batteries were being used since the output required (i.e., the load) exceeded the amperage of the charger.  

Once the lights were off, even with the fridge running, the 10 amp charger would be putting in more than was coming out of the batteries.

Please help me understand the overload condition for just the charger, regardless of its size.  A smaller charger wouldn't keep up with the load, but the batteries would take care of that.  Then with the loads mostly off, the charger replenishes the batteries.

What am I missing, or is it just the heat burnout thing because of the jumper?

Thanks,

Stu

[Mike and Theresa Vaccaro]

Stu,

It was the heat caused by both circuits (10 amps each) running at "full bore" trying to keep up.  Before I bridged the output, it worked fine with only one 10 amp circuit running--it was able to properly dissipate the heat generated, and as you stated, the batteries made up any deficit.

Your energy assessment is almost right on the mark.  (Although I'm probably too old for this type of stuff, there's a 300 watt amplifier powering the subwoofer...tends to draw a few amps now and then...especially when the OTHER amp is on...)

My charger, a Guest type, was optimized for trickle charging a single battery for an electric trolling motor, not charging a 440 amp hour battery bank and acting as a DC power source simultaneously.  Although equipped with a heat sink, it was the sealed, water-proof type and had no integral fan.  We also had it mounted in the wet locker aft of the nav station, so there was limited ventilation.

The real point is to install a charger that's designed for the task at hand!  When I called the engineers at Guest, I could hear the incredulous look on her face over the phone when I described what I was attempting to do with their trolling motor battery charger!


Cheers,

Mike