Adding a second 12v battery

[tbrady]

Hey All!  I'm considering adding a second 12v battery to our 21BHS.  The reason for not going with 2 6 volt batteries is the weight.  I have an Arvika bike rack on the front and 3 bikes which adds about 120lbs of tongue weight already.  To counter that I put a genny, stove, fuel on the back of the trailer and it pulls amazingly well right now. 

By my rough calculation adding a 2nd 12v battery will add around 45lbs of weight vs around 90lbs if I were to switch over to 2 6volts.  I may switch over to 2 6volts once I wear the 12volts out..

Anyway... does it matter if the second 12v is a newer higher capacity battery than what is on it currently?  Any gotchas I should be aware of?

thanks again for all the great help!

[DavidM]

You probably will be fine with new and old 12V batteries.

But I might dispute your thinking about golf cart batteries. Golf cart batteries weigh about 68 lb each, so 136 for two. Two smaller 12V batteries would weigh about 90 lbs, so a 46 lb difference as you note.

But the two GCs give you about 220 amp hours, whereas the two 12V will give you about 140 AHs. If you need the extra capacity, then go with the GCs. In all likelihood, they will also last longer.

David

[tbrady]

Dang, not sure how I screwed that up.  In my "notes" I listed the weight of a single 12v at ~45lbs and a single 6v at ~75lbs.   I must have been looking at a huge 6vlt and not the GC2 which weighs around 60...  Hmm, maybe I'll go ahead and bite the bullet and go 6 volt. 

One problem though is that they welded my battery support only about 7" wide which means I'll need to cut one side off and move it out a bit.

[tbrady]

Oh, one other question David.  How many usable amp/hrs are there with the 6volt?  Is it recommended you only take them to 50% like 12v or can you discharge them a little more?


thanks again!
tim

[DavidM]

Tim:

For best life, say 1,000 charge/discharge cycles, you should not discharge below 50%. But if occasionally you discharge to 80% it won't affect the life that much- hey how many times can you go camping. More importantly, don't leave them discharged long. Recharge to 100% as soon as possible.

David

[Merlin]

So long a both 12V batteries are working well, I agree with DavidM that some age difference is probably fine. However, I would not mix 12V batteries of different capacity. A staged charger like we have in the Camplites can struggle with proper charging if one battery gets full first and needs only float charging while the other battery is still in bulk charge mode.

Why are you adding a second battery? If the answer is capacity and battery longevity and those are really important, then yup, bite the bullet now and go with 2 GC2 6V.

[charliem]

The literature is limited but I have read one article on mixing batteries with different capacities. My take is it will tend to shorten the life of the lower capacity battery causing it to wear out before the larger capacity battery. If you're adding a new large capacity battery in parallel with a low capacity dealer supplied unit it just hastens the demise of the original battery. Probably no great loss but then you have a partially used large battery on hand. If that added battery is sufficient for your needs your some money ahead. If, however, you plan to add yet another high capacity battery to replace the original exhausted dealer unit you're again starting with mismatched units. Save yourself the headaches and go for two new golfers.

[FastEddieB]

This is all very interesting.

My setup has my 12v deep cycle wired in parallel with my two 6v GC2's in series, as long as my selector is on "1+2":



The suggestion seems to be that I might be better just using my 6v's until they deplete a bit, and then let them charge on their own with the switch in the "2" position. And periodically switch to the 12v in the "1" position to let that charge independently.

My habit has been to just leave the switch in "1+2" all the time. That's how it is now with the trailer parked and plugged in to maintain the batteries.

Would be easy to change my SOP if there's any advantage to doing so.