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west4x4

Member Since: 23 Jan 2010
Location: Rutland
Posts: 147

Batteries double or triple?

Just purchased a lithium battery to boost capacity running fridges lights etc camping. Now I already run 120aH AGM in the battery box running through a switched relay system.

The Lithium needs a different charge system which is fine but I’m thinking about the 2 under the seat should I
1) just join the 2 under the seat (they are a matching pair) in parallel. Gives massive capacity but leaving lights on could drain both. But a much simpler system
2) keep the split charge. allows use of the jump function some existing accessories could still run off second battery but extra wiring and could cause issues with rate of charge to the lithium?

Engine is a 300tdi with a standard 65amp alternator

18th Oct 2025 1:07pm

Mdm

Member Since: 11 Sep 2013
Location: Sunny Lancashire
Posts: 1694

for clarity. do you have a standard battery for the car and a split charge to run the agm?
are they both same size and spec and agm ones?

if they are the same then you can join them together minus the split charge without much issue.
if not then its not recommended.

agm need a certain input of amps and type of battery charger to get the best out of them.

lithium need a different charger set up as well.

without the lithium spec its hard to know what it will power and more so what you plan to run off it for how long.
i would have thought that 1 good battery would replace the agm with no problems.

you could also consider a larger spec std alternator and or a 2nd one , (gwyn lewis does a nice kit) sole use to power the aux battery system, this could be 100=140 amp spec as well.

18th Oct 2025 2:35pm

jst

Member Since: 14 Jan 2008
Location: Taunton
Posts: 8480

2011 Defender 110 Puma 2.4 USW Stornoway Grey

If the main aim is extra camping current then don't parallel the first two as then only lithium for camping.

As mdm says ideally 2 different chargers needed for agm and lithium compared to std.

Personally I am running 1750agm as main battery and then 2 x 105ah leasure batteries on split charge for camping. .plus solar controller for leisure Cheers

James
110 2010 XS Utility
130 2011 M57 bespoke Camper
90 2010 Hardtop
90 M57 1988 Hardtop

18th Oct 2025 7:40pm

west4x4

Member Since: 23 Jan 2010
Location: Rutland
Posts: 147

Mdm wrote:
for clarity. do you have a standard battery for the car and a split charge to run the agm?
are they both same size and spec and agm ones?

if they are the same then you can join them together minus the split charge without much issue.
if not then its not recommended.

agm need a certain input of amps and type of battery charger to get the best out of them.

lithium need a different charger set up as well.

without the lithium spec its hard to know what it will power and more so what you plan to run off it for how long.
i would have thought that 1 good battery would replace the agm with no problems.

you could also consider a larger spec std alternator and or a 2nd one , (gwyn lewis does a nice kit) sole use to power the aux battery system, this could be 100=140 amp spec as well.

Current batteries are 2 identical 120ah AGMs with a T-Maxx split charge. Powers 2 auxiliary fuse boxes one under drivers seat powering CB and some cabin lights. One in rear running a Snomaster fridge freezer plus some more lighting. Lithium is 300AH (yes I know it’s overkil) be getting a DC-DC charger for it with some provision for solar. Adding a diesel heater and perhaps looking to run and induction hob instead of gas.

Upgraded alternator has been considered didn’t think about a second one I will investigate that.

18th Oct 2025 9:38pm

MadTom

Member Since: 10 Sep 2013
Location: Olomouc
Posts: 692

No need for bigger alternator. Cars are fine to use electric winch with maximum curren approx. 400A, much more, than alternator is able to deliver. Do not mix alternator max current (A) and battery capacity (Ah). Two lead acid batteries cannot by connected in parallel when the engine is not running (= when they are not charged), they will discharge their self. I use just a big relay switched by ignition, and it works nice with two AGM betteries. "Drobek" = The Small One - Discovery 2, "Blufínek" = The Blue Thing - Defender 130, and for me at least Ford Mondeo

20th Oct 2025 7:01pm

Mdm

Member Since: 11 Sep 2013
Location: Sunny Lancashire
Posts: 1694

that makes it easy.

run the 2 batteries linked (alternator will only see one) , just like a Toyota.

everything else on the lithium.

you can keep std alt or upgrade if you feel it needs it or add a second one for the lithium side of things.

bear in mind a lot of modern vehicle's run 100-220 amp alternators. our old lr dont tend to have smart alternators or big power as std.

a sprinter runs a 180-220 amp version and camper builder/ electrical installers, reckon it can spare 50-60 amp charge for big battery sets up and dc dc chargers.

who ever you use the supplier should be able to advise far better ,as every ones set up is different

20th Oct 2025 7:30pm

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