Dual Batteries

slantt01 · 09-13-2019, 09:36 PM

I've been researching doubling up the house battery in my 2017 Thor 23H. Sites I've been to mention having to use batteries of equal strength, recommending replacing the current used battery with two equally new batteries.

Being retired and on a fixed income I watch my cash flow closely. Two new decent batteries would be a sizable investment. Could someone explain the rationale of needing to use batteries of equal strength?

Any info appreciated.

Jim

taylorbob1 · 09-13-2019, 10:03 PM

Quote:

Originally Posted by slantt01

I've been researching doubling up the house battery in my 2017 Thor 23H. Sites I've been to mention having to use batteries of equal strength, recommending replacing the current used battery with two equally new batteries.
Being retired and on a fixed income I watch my cash flow closely. Two new decent batteries would be a sizable investment. Could someone explain the rationale of needing to use batteries of equal strength?
Any info appreciated.
Jim

Easiest explanation is that the charger makes them both equal.
So if one is old and bad and the other is new and good, the old and bad one will bring down the new and good one.

You can get a 12 volt marine deep cycle battery at Walmart, SAMs, or Costco for about $75-80
If you manage & service them properly, They will last 5yrs or longer.

Bob Denman · 09-13-2019, 10:10 PM

I look at it this way:
In theory: you could hook a Clydesdale up next to a Shetland pony.
But they're not going to be pulling an equal portion of the load: will they?

ducksface · 09-13-2019, 10:37 PM

Use an islolator and don't worry about it.
https://www.amazon.com/Ampper-Batter...-11&th=1&psc=1

Now it doesn't matter equal or not, and if your battery goes dead just manually select the back up battery.
Boats have done this since the beginning of electric on boats.
Dollar wise, this is easiest and cheapest and fail proof.

(finding a place to mount that second battery might really suck)

Bob Denman · 09-13-2019, 10:42 PM

Question: Does the isolator keep them from both being charged at the same time?

Beau388 · 09-13-2019, 10:52 PM

My opinion is if the batteries use the same chemistry, are of the same type of manufacture type (flooded, VRLA, AGM, gel) and are hooked in parallel, it doesn't matter if the size, manufacturer or age matches. It does matter when you hook batteries in series. You will hear overheating, mismatched voltage, parasitic draw stories which are universal but only apply to specific cases not represented here. Hooking flooded or AGM batteries in parallel just adds the amount of lead available for the chemical traction. Only the internal resistances of the different batteries will regulate how fast the batteries charge and discharge. I will admit if you mate a half discharged battery with a fully charged, you will loose more water from the fully charged battery, but you in no way will hurt either battery. As soon as both reach the same charge (internal resistance), they will loose water at the same rate (flooded batteries). Of course AGM batteries do not loose water because of the ability of the hydrogen irons to recombine oxygen ions liberated from the plates when the batteries are being charged. This recombination does occur in flooded batteries but at a much slower rate, so some water is lost. The slower you charge the batteries the less water lost. That is why the better chargers monitor battery temperature rather than the cheaper one assume the batteries are at the same temperature as the charger. It is just simple chemistry, not three dimensional fluid flow.

Beau388 · 09-13-2019, 11:09 PM

I should say that I have a one year old, group 29, Walmart, marine battery mated in parallel with 4 year old, group 27 Harris battery (original) and all is well. The old Harris battery developed a shorted cell after 3 years. The charger is a PPC 5355 with a 4400 temperature assure upgrade (TAU).

MJC62 · 09-14-2019, 03:38 AM

I have gone the one “old” battery paired with a “ new” battery in my battery bank. I have found that all you end up doing is switching out one battery in the bank every other year. When I finally broke down an shelled out the coin for a pair of new ones I could get 4 or more years out of a set. I am retired and on fixed income as well so I know what watching shekels is all about . You can pick up a pair of deep cycle wet cells for the Rv from a Tractor supply for a couple hundred. Change out in a pair, watch the water and 4 years on the batts is not out of the ordinary

DbleDutch · 09-14-2019, 04:27 AM

I have had dual batt in nearly all the configurations noted. Our 5th had dual wet deep cycle with an isolator switch and set at both they never charged well. With an old and new batt as a set one was always dying early. Put a matched set in and got 6 years from them.
I now placed a matched set without an isolator in our 24F and running great a year later. Should see many more years.
NOTE: If your 23H batt bay is like mine and I think it is, place the front batt with negative post forward. It will rest just under the front metal frame. Placed this way will avoid possible shorting to the frame. The positive forward will be risky to accidental shorting when servicing them.

I recommend dual batts, especially if boondocking.

blw2 · 09-14-2019, 11:49 AM

As I understand it....it's not only about "pulling the load", it also supposedly affect them if sitting without load. The stronger battery will always be charging the weaker one.
As lead acid batteries sit, even not connected to anything, they'll slowly loose charge (that I know to be true)...so the old battery might be loosing charge more then the newer one....and so that's why the new battery is always charging the older one.
So as I see it, it's all about maximizing battery capacity and battery life span...
IMO, if you're not trying to save every amp for maximum off grid time, then who cares if it's a little less than optimum.
If it were me, I wouldn't hook up an old battery...especially a cheapo marine/hybrid type to a new and very expensive Trojan golf cart battery.... but if they're both marine hybrids, then I would just do it since I'm not a maximum off grid RV'er.
I like duckface's idea of an isolator to help with the situation... the link is to a switch, that if set to "both" just connects the two batteries together so it's not really isolating...it just allows you to use one battery, then switch to the other when needed...but then you'd have to be sure to switch to both for charging....there are automatic devices I think for this, but prob not worth it for your purposes.
but
that switch or something like it I do definitely suggest so that when in storage you can disconnect the batteries completely to eliminate parasitic loads in storage and to slow the self discharge to harmful levels. First thing I do now is install disconnects on my batteries...I've had to replace way to many over the years form letting the sit in storage too long...

Bob Denman · 09-14-2019, 03:43 PM

It's really about the money; as much as it's simply about not paying more than you have to...

Cheap!

(Just Kidding...)

slantt01 · 09-14-2019, 06:31 PM

Wow!!! Didn't really expect that much response... Thanks to all for your input. I think I'll check out those Walmart batteries.

Thanks again... this is a GREAT forum

J

Bob Denman · 09-14-2019, 06:59 PM

I hope that you find what you need: good luck!

PaulH · 09-21-2019, 07:51 PM

I'm not very technical, but I know it's a bad idea to mix old/new batteries or different outputs. Personnally, I have 2 x 110ah batteries fed by the usual charging as well as 2 x 100w solar panels with a 2000w inverter. This works very well for me when we're not hooked up over 3 or 4 days, and, in my opinion, is well worth the money.

Rocky49 · 09-21-2019, 09:59 PM

After struggling with the original battery in my 2014 Thor 23H for 2 years of ownership, this spring I splurged on a pair of Walmart house batteries a matched pair down to the production date. They fit just fine in the well under the side stairs. Just make sure the cables you buy hook them up in parallel. The extra battery cables at 18" and you need one of each color, get the correct terminals.

The batteries now have the guts to pull in the awning when dry camping.

Rocky

Bob Denman · 09-21-2019, 10:07 PM

Congratulations!

BASSMAN · 09-21-2019, 11:07 PM

I am a retired Mechanic and I have worked on a lot of charging systems over the years, keep your batteries matched, do not mix a group 24 with a group 27 or 29, best to use the same manufacture if possible, I have 6 group 29 deep cycle batteries in my bass boat, 4 of them are for the 24 volt trolling motor, I have 3 solid state chargers, one for the two 12 volt engine batteries and two for the 4 trolling motor batteries, I have 2 groups of 12 volt batteries in series, they are isolated so as I run down one set I switch over to the other set. I had a battery die on the trolling motor and could only find a group 24, that group 24 with the one group 29 would only last a day at best, the other two would go 2 1/2 days, after I got home I swapped out that group 24 for a group 29 and end of problem, I might mention that the solid state chargers for the 24 volt system has 4 connectons, two positive and two negative, the charger only charges 12 volt but it charges each battery separate even though they are hooked in series

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