Auxiliary Battery

Poulsbo1

New member
Battery location beneath passenger seat -- what battery is generally used in this location? Needs to fit - 6 3/4 height, 13 inches long and approximately 8 inches wide. I tried to use a similar battery as in the rear port-side.
 

bl_smith25

New member
Sounds like you have a 2006 with the battery under the floor at the back. It is not a good location to add another battery under the passenger seat with this setup since the two batteries are so far away. They'd need to be two different sizes and the proper cabling between two different batteries at that distance is too complicated to get it right for equal charging and discharging.
Consider adding a battery over top of the one under the floor. Set it on the floor under the couch as far forward as the wood cabinet will allow. From there, the wiring is easy and you can do either two 6V in series or two 12V in parallel and get the wiring right. You MUST use AGM batteries for both of course. The original group 24 battery under the floor was AGM as it's considered to be in the living quarters and if you add one on the floor under the couch, it certainly is.
I have a group 24 in the box under the floor and a group 31 on top of it wired in parallel. While not ideal due to the mismatched sizes, the cabling and connections are short and easy and correct. They've been working well for probably around 5 years now.
 

db34803

New member
Sounds like you have a 2006 with the battery under the floor at the back. It is not a good location to add another battery under the passenger seat with this setup since the two batteries are so far away. They'd need to be two different sizes and the proper cabling between two different batteries at that distance is too complicated to get it right for equal charging and discharging.
Consider adding a battery over top of the one under the floor. Set it on the floor under the couch as far forward as the wood cabinet will allow. From there, the wiring is easy and you can do either two 6V in series or two 12V in parallel and get the wiring right. You MUST use AGM batteries for both of course. The original group 24 battery under the floor was AGM as it's considered to be in the living quarters and if you add one on the floor under the couch, it certainly is.
I have a group 24 in the box under the floor and a group 31 on top of it wired in parallel. While not ideal due to the mismatched sizes, the cabling and connections are short and easy and correct. They've been working well for probably around 5 years now.
Hello Bl_smith25, we have a 2006 Sprinter / 2007 Pleasureway conversion with the house battery (Grp 24) under the floor at the back. I need to increase the house battery capacity from the Grp 24 ~75 amp hr to something over 100 amp hr to be able to run a cpap overnight (7-8 hrs) without drawing down the battery too far. I've been thinking about replacing the battery box in its current location with one that is large enough to hold a Grp 27 or Grp 31 battery. There appears to be enough room to fit a box for a Grp 27, maybe even a Grp 31. But this means removing the current battery box which is welded to the frame (a knarly task) and installing a new one for the new battery. I like your idea of installing a second battery on the floor above the current one. You give up some under the couch storage but keep the re-cabling simple. Would you post a couple of pictures of your installation? I'd like to see your cable runs and how you've secured the battery box. TIA
 

borabora

Well-known member
Have you considered the option of leaving the current system as is but adding a separate LIFEPO4 system just to power the cpap? If you think that a 100 ah conventional AGM is good enough then you need about a 50 ah LIFEPO4 battery, possibly less. You could build your own system with charger, battery and inverter (if needed) or buy a complete "power station." for the purpose.
First step would be to asses if your cpap runs only on 120 AC or can natively run on 12V and then measure how much power it needs for a long night. Second step is how to charge the system during the day -- do you drive, boondock or plug in?
 

db34803

New member
The cpap can run directly on 12v. According to the data plate it draws 6.7 amps on 12v, so I figure that it will draw 47-54 ahs overnight; hence the need for something over a 100 ah battery to avoid drawing it down too far. I don't follow how a 50 ah LIFEPO4 battery can provide enough power. Ah are Ah whether they come from an AGM or a LIFEPO4 battery?
My preference is to keep it simple. I'd like to have just one house battery with sufficient power to run the cpap overnight if possible. This is all for when we are boondocking. If we have shore power there is no problem running the cpap overnight. Plan is to run the generator during the day to recharge the house battery if we are staying put. Shouldn't be an issue if we're driving for a couple of hours the next day.
 

tinman

Well-known member
I think that Borabora is referring to the different performance characteristics of AGM vs. LIFEPO4. You can only draw down about half the rated capacity of the lead-acid battery before its voltage drops and you significantly shorten the life of the battery, while the lithium battery has pretty much 100% of its capacity available. Either way, if you're charging while driving on the same charging circuit as your chassis battery, you won't come anywhere near to replacing 50 or so amp hours in a couple of hours of driving. You would need to consider a B2B charger to get anywhere close to that.
 

borabora

Well-known member
While I generally recommend LIFEPO4 systems over lead-acid, it would be a lot easier and cheaper to just parallel a second 75 ah battery with your current one -- or replace them both if the existing one is worn at all. Going LIFEPO4 would give you a more stable long-term solution but you'd need a DC-DC charger and deal with wiring changes. If your cpap draws a continuous 6.7 amps then I would recommend at least 150 ah for lead-acid or 100 ah for LIFEPO4. But the 6.7 amps might be peak draw and the device may average less -- I don't know. It would be worthwhile to measure because if the average is less then a 100 ah battery could be enough or a self-contained unit might be enough at an acceptable price.
 

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