Any one running dual batteries?

[edgeau]

Long story short: my gas fridge died and I have replaced it with a 12v compressor type. I am currently running an experiment to measure it's real world power use. See http://tnttt.com/viewtopic.php?p=1269247#p1269247

I might well find I need more battery. The problem is my current (pun intended) one is still good so I don't want to ditch it.

Two batteries in parallel seems a potential solution.

Possible downfalls?
Space. They could well end up at different ends of the trailer. Hmm good heavy guage wire may help but even so...
Mismatch capacity
The charge controller has no dual battery smarts so it will treat them as one. If they have different capacities will that degrade the life of one?

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[John61CT]

The issue is not paralleling different sizes so much.

But pairing old with new will greatly shorten the new one's lifespan.

If you're just talking a cheap $200 FLA unit no big deal, but if a fancy $500 AGM I wouldn't.

The best battery value by far is Duracell (actually Deka/East Penn) FLA deep cycle golf cart batteries, 2x6V, around $200 per 200+AH @12V pair from BatteriesPlus or Sam's Club. NAPA relabels it here: https://www.napaonline.com/en/p/NBP8144 Deka self-labeled also sold at Lowes.

All this is US-specific, overseas can be double or triple for true deep cycling lead, TrojanS are a decent alternative to the above Deka, T-105 for example.

AGMs don't last as long and do cost a lot more per Ah, so only go that way if necessary.

[John61CT]

sorry site glitch duplicate post

[John61CT]

PS "dual battery" most often means House vs Starter banks, with a combiner / VSR/ACR device in between.

Two series string batteries in parallel are really just one bigger battery electrically.

[saltydawg]

The next question is what size is the current battery? do you have solar? how long will you be out with out shore power" how often will you be moving, ie will you charge from the tow vehicle?

If you have a 100 ah battery you can always add solar to keep you going. with a fridge and normal usages 2 days with out charge on a lead acid battery is pushing it, but 200 to 300 watts of solar ( as in in actual sun no shade ) would keep you going for really as long as you want.

The issues with 2 batteries esp if different capacities come with over charging the lower one. Also if on different ends of the trailer, as you said you need a big wire because any voltage drop will also mess with the charge.

another option is a new battery, lifepo4 batteries have twice the useful capacity for the size, so where you could fit a 100 ah lead battery that you can only use 50 amps, you could put a lifepo4 and have 120 amps of usefull power.

[RJ Howell]

I am surprised how little my frig actually uses! You direction is very good, test in real life! I tested in my Dining Room and results were pretty close to actually in the camper.

I ran duals with my commercial camper, but with a smaller unit (lack of storage) I bought a LiFepo4. I found a great deal on a 60ah and tossed the dice. It has worked out well for us. I mainly run the frig and charge phones from the house battery. Doing this I'm 2.5 days before requiring a re-charge. Since we drive on in a couple days, the DC/DC charger works very well to keep us powered up.

Like Salty asks (and for good reason) the questions. There's a lot of weight to running duals and if so, buying new, matched should be the way to go. I'm not a fan of dual 12v's, to me it misses the pint of running duals, rather I ran dual 6v's. Deeper cycle, thicker plates, actually meant to be slow drawn.

If you can afford, go LiFePo4. I don't mean go out and buy a Battle-Born.. research a bit and maybe built a unit.

[edgeau]

The next question is what size is the current battery? do you have solar? how long will you be out with out shore power" how often will you be moving, ie will you charge from the tow vehicle?

.

Salty, yes all those details are in the build thread but the short answer is
40Ah battery 100watts solar, never have used shore power, stay the one spot for a week or so and no charge from the TV.

The new fridge has potential knock on for not just the battery but the solar and charge controller too. I am still running the experiment but was pleasantly surprised the fridge only used 2.6Ah over 9.5hours last night on "Eco" mode. It is still cool spring weather here though and I will have to make allowances for running at the height of Summer.



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[saltydawg]

Okay with 100 watts of solar and the draw of 2.6 amps over 9.5 hours thats .27 amp hours per hour,or 5.6 ah's a day. I am assuming the battery is lead acid and if we bump the summer fridge load to 10 amp hours a day for safety. If you get 80 watts ( probably closer to 70 ) actual from your panels and 5 hours of sun your putting about 30 amps a day in to your battery, so you can recharge everyday from just the sun. Unless your running a lot of lights, and a fan all night you should be okay.

My vote is a 30 to 60 amp hour lifepo4 battery, which should fit in the same place your battery is now. If your not running a mppt charger on the solar do that plus if possible bump the solar, the mppt charger will give you 10 to 30 % more charge. And if you have room on the roof for more solar get another 50 to 100 watts up there and you should have no problems.

What are the rest of your loads? Lights, fans, radios etc?

Edit the reason I say get a "bigger" battery is to buy your self an extra day or two for cloudy days or if your in the shade one day. The mppt charger will help with the shadey days as well.

[edgeau]

Yep. Running an mppt. More solar is on the agenda. I just hate throwing out things that still work hence wanting to keep the battery and supplement if possible. It has only done about 3 weeks work.

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[MickinOz]

Edge, I reckon your current system will be stretched but not destroyed by your new fridge.
Going by your experimental results so far, and remembering you live in Sunny Queensland, with a 100 watt panel day time usage should be looked after by the panel with some battery charging capacity left over and night time use shouldn't get too tough on the battery.
A 100 watt panel is really 80 watts after allowing for charging losses etc. Say 6 amps. At 6 or 7 hours full sun in Queensland, not an unreasonable assumption, you should still be able to put a full charge in your battery every day.
When/if your battery does die, you could then switch to lithium.

It has come to my notice, via a very unscientific survey of Ebay, that the price range of 40-50 AH Lithium Iron Phosphate batteries is overlapping the price range of 100AH lead acid here in Oz. i.e. the price of a low end 50 AH lithium with in-built BMS is now $250.
Not exactly chicken feed, but 50 AH lithium effectively doubles the usable power, compared to your current 40 AH lead acid.
Assumes your MPPT can be set for lithium.

So my vote is use what you got until it proves inadequate, then replace with lithium.

Edit: I see my logic roughly parallels that of Saltdawg. Must be right then.

[edgeau]

Success. We are on a trip now and ran the fridge, his and hers fans, charged 4 phones and a portable battery pack for phones last night. Battery still at 56% this morning. So definitely need daily charging but it is back up to 70% at 930am on a cloudy day. . Happy camper

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[saltydawg]

Congrats, thats just about what mick and I thought. But summer heat will add up in heat losses on the solar panel and more load on the fridge.

get more solar.

[Pinstriper]

If you really want to make productive use of the old battery, get an A/B switch and you have a failover option. An extra 30ah when in dire need, such as something going wrong with your primary battery while on a trip, or running low on your primary battery before you can get some daylight into it, get you through the night.

If you have the space, that is.

[pchast]

I run a pair of 35ah batteries (same age date) with a 40watt solar panel and a cheap MPPT charge controller. Over the last 6+ years they have had light use and still seem to take a full charge. When traveling in the past I connected to the car through the solar charge controller for maintenance charging.

[TimC]

I run with eight 18aHr AGM batteries because I didn't want to have a large volume space dedicated to batteries. Total is 144 aHr. I have "hidden" batteries tucked into some really small areas that were kind of wasted space. Yes, 144 aHr at 18 aHr each did cost a bit more per aHr than just going with two larger batteries, but, they did give me more options to use space that might have otherwise been wasted. Wasted space in a teardrop is a sin!

Regardless of where the batteries are located I've made sure the wire length from battery to battery is similar. I do have four batteries from 2016 and four from 2017. They have all seemed to balance out holding the same voltage. I keep them on a charge controller 365 days a year and believe that that will give me extended service life. We'll see.