Battery isolator

[dan filipi]

I wanted to get your guys' opinion on this.

In my service van I have a second battery to power a 120 volt invertor.

I've been charging it connected to a fuse panel under the driver seat, charge would only take place when the ignition key is on but after the main battery died after just 6 months I've been monitoring the charge and voltage at this second battery.
I was finding with the newer alternators that once the main battery comes up to charge the voltage would drop off to 13.6-13.8 and when the second battery is already drained from using the invertor the alternator and the where I had it connected at the fuse panel wasn't supplying a high enough voltage and amperage.

So what I've done is run a 4 gauge wire from the second battery to the main battery and installed a battery isolator.

(I found a Sunpower unit for $35)

https://www.grainger.com/Grainger/BATTER ... ator-4NGT5

What it does is senses the main battery voltage, once that voltage is above 13.8 it closes a relay to charge the second battery.
That seems to be working perfectly.
What it also does it keeps the relay closed until the main battery voltage drops to 12.8 then the 2 batteries are uncoupled.
My concern here is the second battery is a Optima yellow top, expensive but worth it. May last one lasted 10 years!
It's full charge voltage is 12.99 so what happens is while the 2 batteries are coupled it's actually draining the Optima down to 12.8 because the full charge voltage of the main battery is 12.8.

Maybe no concern here but I'm wondering if by making the second battery "cycle" from full charge down to 12.8 I'm actually reducing it's life or is that so minimal I'm being too anal?

 

[joedrum]

me dont know ...but it looks neat ...and probably dose work the wat you said

 

[mcgovern61]

Yes...the lower voltage battery will keep bringing down the higher voltage battery. We use these systems on boats and ships all of the time. We may have 3 different battery sources in 3 locations that can take their charge from 3 different charge sources (normally from the 2 main engine alternators and an AC powered battery charger). We always employ what is known as a best battery system where there are diodes (isolators) protecting each power source as well as each battery bank (normally 3 banks of 24 VDC - two 12 VDC in series ea). The best battery system senses the voltage of any battery and charges it from the highest available source. All of the battery banks are to be maintained at a maximum 28.8 VDC.

Your issue is that your batteries apparently have two different maximum voltages. The alternator will sense the low voltage and put out until any battery connected reaches the maximum voltage. The higher battery will also provide some voltage and will drain down accordingly. In marine, we can regulate the maximum voltage and maintain that.

 

[toytender01]

12.8 or 12.99 I think you are good to go :salute: :salute:

 

[dan filipi]

12.8 or 12.99 I think you are good to go :salute: :salute:

Yeah I'm prolly just being too picky.

The agm costs over $150 so I'm trying to make sure it lasts.

 

[slabghost]

For peace of mind you might want to put a battery charger on the agm and top it off completely every once in a while. I agree it probably is good as it is. But I'm not a battery technician.