6v AGM external charging

[Joel Ashley]

I’ve read about it here before but forgot how it’s best done.  I want to use my portable charger to boost back the battery banks because after replacing my two 12v chassis ones a year ago, and then soon after that the four old wet cell house set with AGM’s, our inverter decided to go belly up.  As many of you know, our coach has been parked by the house here for 5 years due to our multiple health issues.  Usually the solar has kept the battery systems healthy, but switching batteries started a negative and disruptive scenario.

The inverter, or at least it’s remote, was already acting up when a squirrel snuck into the temporarily open inverter’s bay last summer, and tore up a thin plastic picnic table cover.  The next time the inverter ran that loosened plastic got sucked up over the inverter’s rear vents apparently causing overheating.  According to Magnum, that alone would’ve just required my replacing an internal heat protective module of some sort. 

But then I screwed up big time.  Finding the remote reporting things working fine for awhile, and then not, I opted to try an inverter Reset by holding the button on the inverter for several seconds.  Oops.  I didn’t realize, or had forgotten since I’ve hardly ever needed to use that reset technique, DO NOT press that button if you’re still plugged into shore power!

Now the inverter won’t do anything, its remote reports nonsense terms or is totally off, and the whole system seems dead.  No lights on the inverter or response even when unplugged from shore or genset running.  The genset relies (I think) on the inverter’s charger, so running the genset won’t bring back the batteries, and the chassis set is 12.5v;  the house AGM set though is below 12v.  Apparently the solar side hasn’t kept either battery bank up to snuff (I should’ve had the Main switches off). 

So how do I hook up my portable charger to the AGM set, four 6v in standard configuration to supply 12v?  Should I disconnect any cables to them first or are the BIRD, undamaged inverter components, and other peripherals safe for external charging fully connected?  Should the charger be set for 2amp, 15amp, or is a higher charge rate okay?

Joel

[Gene Obie]

I think I would disconnect shore, turn off both coach/chassis isolation switches and remove the negative leads back to the coach/inverter from the 4 coach batteries. Then just attach your charger to coach battery + / - (leave the 4 coach batteries in their series/parallel connection). That way you wouldn't be able to supply any additional current back to the inverter and will bring up all 4 batteries  together. You should be able to charge at C/3.. that's probably much higher than your external charger can do (excluding boost modes). Also, some folks recommend using a large power resistor to charge the inverter caps when you go to reconnect the leads to power up the inverter (to avoid the big spark if disconnected for long time) - i have a 15ohm/50W I got from Amazon for this purpose. Just a few seconds running power thru the resistor to limit current than direct connect (caps stay charged for some time).

[Fred Brooks]

    Joel,
Isolate the new AGM batteries from the coach. (Disconnect the ground cable and the positive cable) Connect your independent battery charger and trickle charge the bank for 2 or 3 days to bring them back to 12.8. With the shore power cord not plugged in, Check the voltage at the Magnum battery terminals. If no voltage, check the in-line 200amp fuse that powers that battery cable. If you do have 12.8 volts at the Magnum, disconnect the positive cable for 5 minutes and then reconnect. (You will get a large spark, but this is normal). Remove your remote panel and unplug the RJ-11 plug from the back side. Reconnect and the remote will reboot and identify what it is connected to. If this does not resolve your issue, you may have to replace the magnum inverter/converter. Hope this helps, Fred