Using two batteries in the TD

[pete.wilson]

Hey

New to the forums.

I have 2ea 12v trolling motor batteries (242min of reserve capacity ea.)connected in parallel with no isolation between them. I use this in a slide- in box that goes into the back of my suburban and it powers a 2200Watt inverter that I use for powering a microwave oven, toaster oven etc. And after a year of use I have very little problems with degraded capacity. I would recommend a disconnect from the battery to the inverter if stored for long periods. I do use a small 1.5amp trickle charger to top off when I go do my field research. Use the largest capcity batteries you can to give the longest run time, the inverter will drop out when battery voltage reaches about 10.5V. Both batteries must be the same...important.

Pete Wilson
Kansas Bigfoot Investigations

[ferociousllama]

How come no one has suggested two 6 volt batteries in series in a trailer? It will give you more than double 2 12 volt batteries. It wont cause any drain because the batteries act as a single battery. I have a lot of friends who use four 6v batteries and their batteries last much longer than people with four 12v batteries.

[Leon]

How come no one has suggested two 6 volt batteries in series in a trailer? It will give you more than double 2 12 volt batteries. It wont cause any drain because the batteries act as a single battery. I have a lot of friends who use four 6v batteries and their batteries last much longer than people with four 12v batteries.

Did you read alanm's post on the previous page? "I have preferred to use 6 volt golf cart units so I make pairs of them in series for 12 volts and then join the pairs in parallel for the amphours. "

[brian_bp]

How come no one has suggested two 6 volt batteries in series in a trailer? It will give you more than double 2 12 volt batteries.

That doesn't make any sense to me. Batteries come in all sorts of sizes, and you can't just count them. Also, all lead-acid batteries are sets of 2-volt cells in series (and in some configurations some more sets in parallel with that), so no matter how many boxes you divide them into (one box is 12V battery, two boxes are each 6V batteries). Two six-volt batteries in series are just easier to carry around than one 12V battery of the same capacity and construction.

I have a lot of friends who use four 6v batteries and their batteries last much longer than people with four 12v batteries.

And I know of people who drive Kenworths who find they last longer than Ford pickups... but it's not the size, they're just not comparable products. The 6V batteries used by most people are heavy-duty deep-cycle units, and the 12V batteries they are compared with are commonly cheap consumer products, often not even intended for deep-cycle use.

There is also the legitimate issue of balancing paralleled batteries, as discussed earlier.

[pete.wilson]

Hey

He's right though, that using 4ea 6v batteries connected in a series/parallel fashion does have more capacity and therefore last longer. There is an addition issue of additional mounting requirements, added space and more sophisticated wiring which makes some people nervous. The traditional 2ea. 12v trolling motor deep cycle battery can be purchased almost anywhere (Walmart) and is relatively cheap. It really comes down to a combination of price, availability, simplicity in setup, weight. It's just personal choice and what your familiar with.

[brian_bp]

Mike is asking about paralleling the Jeep and trailer batteries, and the concerns with doing that have been addressed (personally, I would connect them for charging, but not draw on the starting-type Jeep battery while camped).

That just leaves the more general question of building big battery banks, and what voltage of battery to use. I'll take one more shot at this...

He's right though, that using 4ea 6v batteries connected in a series/parallel fashion does have more capacity and therefore last longer...

More capacity than what? If you compare four 6V industrial-duty deep-cycle batteries (connected series/parallel, as required to make a big 12V bank), to any number of 12V batteries in parallel, if the total size of the batteries (just add up their weights... seriously) is the same, the total capacity is the same - not more - so the only reason for a difference in lifespan is balancing of charging. If you compare four 60-lb batteries to two 60 lb batteries then yes, that's more capacity and they should last longer (and be more efficient).

More capacity does mean that any given rate or depth of discharge is a lower fraction of the batteries' capacity, so they last longer. It just doesn't matter how the pile of lead plates is split up...

four GC2 (Trojan T-105) in series/parallel:
225 A-h @ 6V, 62 lb each = 450 A-h @ 12V, 248 lb total

four Group 27 (Trojan 27TMH) in parallel:
115 A-h @ 12V, 59 lb each = 460 A-h @ 12V, 236 lb total

two Trojan L16H in series:
420 A-h @ 6V, 121 lb each = 420 A-h @ 12V, 242 lb total

two Group 8D (Trojan EV8D-2) in parallel:
216 A-h @ 12V, 132 lb each = 432 A-h @ 12V, 264 lb total

It's a real pain to lift the L16H or 8D batteries, so it's nice to break it up into smaller boxes, but the total weight and electrical capacity are the same in each combination. These are all deep-cycle batteries from the same series of the same (very well-known) manufacturer, all rated by their 20-hour discharge capacity. The big batteries have a slight weight disadvantage, because they need very strong cases.

alanm's scheme is a common and perfectly legitimate way to get the total voltage and capacity required, as he explained. It also uses a readily available size of battery (a 6V unit commonly used for golf carts, such as the T-105), which most people can lift into place; that's the size I used for my examples above. These are good things, but it's not the only way to get there, and I'm not sure why a typical teardrop or tiny travel trailer needs so much capacity that it has to be in four boxes of lead, totaling 240 lb and 450 A-h!

So if you can't get enough battery, cheaply and readily available, in one box you can lift into the trailer, then sure... combine some. Series is easier to manage, although if one dies you have nothing (instead of half capacity), but both ways work - there's no magic to any particular battery voltage.

[xccelagator]

Wire the batteries in parallel and switch there position around if you want. I would not worry about this battery and that battery discharging or charging more than the other one. Wired parallel they will equalized to a extent. You are using the batteries not the other way around.

Yes depending on the wiring setup one with discharge faster than the other and discharge etc ect etc. I raced RC cars with sub Cs and yes there was one cell on the positive or negative end that faired better. I can't remember. Your enjoying camping not living on the moon guys!!!!

Golf cart batteries are great for extended stays, but dang one 6 volt is 60+ lbs times 2!!!!!!

Dan

[brian_bp]

Wire the batteries in parallel and switch there position around if you want....

I raced RC cars with sub Cs and yes there was one cell on the positive or negative end that faired better...

Those RC packs are typically straight series sets, not parallel: 6 cells for 7.2V, 8 cells for 9.6 volts.

I do agree that there must be some compromise between the electrically ideal and the functionally practical for enjoyment!