3 replies to this topic

[Jzack605]

I’m about to add a second 12v battery but I’ve seen a few indicate that 2x 6v batteries is a better set up. Could someone explain why this would be better to an electrical noob?

[ckent323]

Jzack605,

 

I will assume that you are asking relative to some type of lead acid battery rather than Lithium.

As far as I am aware there are few true deep cycle 12v batteries available.  I am most familiar with Lifeline.

Their largest 12v true deep cycle battery that would be practical for a camper (group 31 size) has a rated capacity 20 Hour rate of 100 Ah and weighs about 64 pounds).   There are much larger 12v deep cycle batteries but they are very heavy (over 100 pounds and too large to fit in a camper). Their equivalent size (group 31) 6v true deep cycle battery has a rated capacity 20 Hour rate of 220 Ah and weighs 66 pounds.  Again there are larger 6V batteries but their size and weight make using them in a camper impractical/impossible).

So you can get roughly double the available Ah by using two 6v batteries.  But if you use 2 12v deep cycle batteries in parallel you can also get about 200 Ah.

Two 6 volt deep cycle batteries must be wired in series (positive to negative) to get 12 v with 220 Ah rated capacity. Each 6 volt battery pair operates as a single 12 volt battery. 

In general, I  prefer two 6 volt AGM over one or two two 12 volt AGM batteries. Typically the two types are made with different thickness plates and will perform differently. It would be best to talk to Lifeline about the differences between their 12v and 6V true deep cycle batteries to understand the differences and which would be best for you.

Two quality Lifeline 6v AGM 220 Ah batteries will cost around $700. Two quality 12v AGM 105 Ah batteries will cost around $900. A pair of either of these will provide around 100 - 110 Ah of usable power (see advise items below).

You may instead want to consider a quality Lithium Ion battery which will provide 100 Ah with about 80 Ah usable power and about 1/2 the weight of going with Lead Acid AGM.  However, if you do not already have charging electronics to accomodate a Lithium Ion battery you need to factor in those costs too. 

 

Keep in mind that it is generally advisable to do the following with 12v lead acid batteries (FLA, AGM or Gel).

1) Monitor the battery state of charge using a shunt based battery monitor (such as a Victron BVM-712 or equivalent).  This is true for Lithium Ion batteries too.
2) Do not routinely discharge below 50% capacity (discharging below 50% routinely will lessen battery life).  Lithium Ion batteries can be routinely discharged to about 20% of full charge.

3) Always fully recharge the battery before discharging again (this refers to any partial discharging) to minimize sulphation of the lead plates.  Lithium Ion batteries don't care if they are partially recharged before the next discharge.

4) Generally all batteries in a battery bank should be the same brand and type as well as close to the same age.  This is true for Lead Acid and particularly true to Lithium Ion.

https://lifelinebatt...s/rv-batteries/


Summary:  I suspect your existing 12 v battery is likely not a high quality deep cycle battery and may have enough age that you may have to replace it in the not too distant future. 

Given the decreasing cost of Lithium Ion batteries now as well as the lower weight and more usable capacity you might want to consider changing to a Lithium Ion battery bank (with appropriate charge controller and battery monitor - I like Victron).  A complicating factor is the truck to camper connection for lithium ion - you may need to add a DC-to DC charger as well as breakers at the truck battery and the camper battery since your vehicle battery is probably lead acid and if you have a newer truck it may have a smart charging system.  There are several threads on WTW on doing this and it is too much to go into here.

I hope this is helpful.


Craig

Edited by ckent323, 24 October 2023 - 07:07 PM.

[Vic Harder]

Excellent write up Craig!

[Jzack605]

Wow thank you Craig!