Adding an extra battery to my RV

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timtune

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I have a 98 Roadtrek and even with a brand new "shore" battery we can barely last 3 days boondocking - and that's to run our water pump and sensors only - during those times we use separate battery powered lamps at night.

I finally realized there is space underneath for a battery where a factory genset was supposed to go. Could it possibly be as easy as just locating the battery and then fabbing two cables to connect it in parallel?

Would a second battery effect how it charges??
 
timtune said:
I have a 98 Roadtrek and even with a brand new "shore" battery we can barely last 3 days boondocking - and that's to run our water pump and sensors only - during those times we use separate battery powered lamps at night.

I finally realized there is space underneath for a battery where a factory genset was supposed to go. Could it possibly be as easy as just locating the battery and then fabbing two cables to connect it in parallel?

Would a second battery effect how it charges??
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Both should be about the same age and size. We have 2 130 ah agms and a 300ah li iron battery on ours the lithium runs the AC only. Fridge and lights etc are on the agms. We also have 200w of solar to charge while sitting still.

Sent from the future
 
I wouldnt hardwire the batteries in parallel. I would put in a selector switch that allows you to select one to use. Add a diode battery isolator to allow alternator to charge both batteries. With both batteries hardwired in parallel you can have issues with one dying early (especially if they aren't a matched pair to begin with). My way costs more to wire but you can have two different batteries with no issue (other than figuring out that battery A is good for 3 days but battery B is good for 4).
 
With both batteries in parallel if one dies early then I'm still connected to the second. Would I not have 12V until both batteries are dead?
 
timtune said:
With both batteries in parallel if one dies early then I'm still connected to the second. Would I not have 12V until both batteries are dead?
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The good one will keep the bad one propped up. They will both run out of power at the same time. Worst case scenario, with one new battery and one crap one, the good one will try to charge the crap one and both could be dead in a few hours.

The switch keeps them isolated (and easy to turn off). You can run battery A until dead and then know Battery B should be good for another 3 days or so . Or if a pump sticks on and kills A quickly, B will still be good after you fix pump. Obviously if you have used A and pump sticks on with B, you are dead.
 
Also never let them get below 12v or they will be garbage very fast. And yes 2 new would be better a suitcase style 300 a lithium would last much longer as long as you don't use it in the winter.

Sent from the future
 
timtune said:
I have a 98 Roadtrek and even with a brand new "shore" battery we can barely last 3 days boondocking - and that's to run our water pump and sensors only - during those times we use separate battery powered lamps at night.

I finally realized there is space underneath for a battery where a factory genset was supposed to go. Could it possibly be as easy as just locating the battery and then fabbing two cables to connect it in parallel?

Would a second battery effect how it charges??
Click to expand...
Thats correct, that easy..remember to install new batteries as adding a new battery to and old will only garner as much as the old battery.
 
timtune said:
Can I avoid the problem by starting with 2 new batteries??
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Are you using lead acid or AGM at the moment and how many amp hours is your battery? You may not be interested in changing chemistries but have you thought about going LiFePO4?

Ever since switching to lithium iron phosphate for my off-grid cottage and portable 12v needs I'll never go back. They don't like temps below freezing while charging/discharging however, so it may not be for you depending on your needs (but you can always add heater pads to your packs to keep them above freezing while in use).

One of the first packs I built was a 280Ah 12 volt pack for portable use - gave me double the usable power and was half the weight of the battery it replaced! If you're always running out or worried about power usage while boondocking switching to LiFePO4 will help out immensely.
 
Xhumeka said:
Are you using lead acid or AGM at the moment and how many amp hours is your battery? You may not be interested in changing chemistries but have you thought about going LiFePO4?

Ever since switching to lithium iron phosphate for my off-grid cottage and portable 12v needs I'll never go back. They don't like temps below freezing while charging/discharging however, so it may not be for you depending on your needs (but you can always add heater pads to your packs to keep them above freezing while in use).

One of the first packs I built was a 280Ah 12 volt pack for portable use - gave me double the usable power and was half the weight of the battery it replaced! If you're always running out or worried about power usage while boondocking switching to LiFePO4 will help out immensely.
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I enquired about this before and I recall a number of people being concerned that switching from lead acid may cause trouble for/with my charging system. As well as concerns of batteries overheating I think it was.
 
timtune said:
I enquired about this before and I recall a number of people being concerned that switching from lead acid may cause trouble for/with my charging system. As well as concerns of batteries overheating I think it was.
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Lithium Iron is a different animal than Lithium Polymer or Lithium Ion. Lithium iron is a good fit for long low discharge. It may have happened but I haven't heard of Lithium Iron (LiFePO4) spontaneous combustion.
 
timtune said:
I enquired about this before and I recall a number of people being concerned that switching from lead acid may cause trouble for/with my charging system. As well as concerns of batteries overheating I think it was.
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I don't think you'd have any _extra_ issues with overheating compared to lead acid/agm - as long as you're not leaving the batteries in direct sunlight or beside another heat source I'd think you should be fine. And yes, as GreyGhost points out iron phosphate is much safer than the other lithium technologies.

As for charging, the new BMS (battery management systems) are very effective at accepting any sort of charge and cutting off charge if any particular cell rises above a safe level. I just use an old 12 volt charger i got from Canadian tire (the 25A CAT rebrand type) for my portable 12v and it works fine. In fact any charger I have used has worked fine, solar chargers, 120volt chargers, etc.

But all this being said, you should definitely do your research for your particular application if you are thinking of switching. The DIY Solar forums have been an invaluable source of LiFePO4 info for me, and they have a section devoted to vehicle mounted systems. Very friendly helpful community too - check it out if you want to research a bit further.
 
Best bet is some type of isolator to isolate both batteries from each other (the right way). Even two new ones will not be perfectly matched so the sum will not be the sum of the parts in parallel. It may work but it is not ideal and lifespan may also be degraded. Really mismatched could be problematic sooner.
 
I have a 60 amp DC DC charger in my system isolates everything very nicely no worries about back feeding or anything else

Sent from the future
 
PM me I have a 100 amp suitcase style lithium iron battery you could borrow for a week or a weekend just to see the difference it will probably outlast your AGM's two to one

Sent from the future
 
Be careful mixing chemistries in batteries without an isolator , and be sure your charger can manage different chemistries at the same time . Some need to be set for a battery type .
Doing a load calculation for what you require to run is important, so not only do you know what you use , but also what you don’t use . We had three batteries in our RV , a starter battery that only started the engine , separate from everything. And two deep cycle flooded conventional batteries , I went with deep cycle group 31 to after pulling two huge G2 golf cart


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Thanks all. I decided to just wire up the second lead acid - I'm welding up a rack to hold it underneath now.
I'm going to buy a new battery. The current one was new last summer.
It may be all I need - time will tell. Thanks for the input.
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