• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

Dual battery options

Gerhard van Rooyen

Staff member
G-Wagen Club SA
Joined
Jan 21, 2009
Messages
509
Likes
37
Age
33
Location
Empangeni/Durban
Gerhard, you are correct but initial charging could be higher than 20A
OK, from all articles and peoples comments I understand that more than 20A is not good for lead acid and thus the use of constant 20A step chargers?
I think maybe I will see how long this battery lasts being charged with the alternator before I spend R1k-R5k on a charger.
 
Joined
Jul 9, 2010
Messages
577
Likes
65
Location
Johannesburg
OK, from all articles and peoples comments I understand that more than 20A is not good for lead acid and thus the use of constant 20A step chargers?
I think maybe I will see how long this battery lasts being charged with the alternator before I spend R1k-R5k on a charger.
I would expect your battery to last a decent period of time. For many years before dc-dc chargers were available pretty much all dual battery installations were simply a solenoid which connected the second battery in parallel to the main battery. I've had several systems like this and most of the time the beers stayed cold on trips.

The biggest benefit of the dc-dc chargers is that they charge the batteries faster, meaning the batteries more often reach a fully charged state. Imho a dc-dc system is the better way to go, but if the finances isn't there to buy one, don't stress about it - install a simple solenoid based setup and use the money you saved to go explore this beatiful part of the world we live in.

For a solenoid based system, make sure you use proper thick cables for the installation to eliminate as much of the voltage drop as you can. Ensure everything is properly protected with fuses (at both batteries) to prevent your vehicle going up in smoke.
 

Gerhard van Rooyen

Staff member
G-Wagen Club SA
Joined
Jan 21, 2009
Messages
509
Likes
37
Age
33
Location
Empangeni/Durban
Thanks Paul

That is exactly my plan!

Just one question remain - should there be a fuse in the charge circuit (thick cables) and if so which capacity?


* I have lent an Ampere meter and will measure how much amps the second battery draws and report back.
 
Joined
Jul 9, 2010
Messages
577
Likes
65
Location
Johannesburg
Gerhard

Yes - I would recommend a fuse on both ends of the (positive) charging cable, as close as practical to the battery terminal. Remember - the current for a short can come from either of the two batteries, hence the need to have a fuse on both sides.

The main purpose of the fuse is to protect the wire from overload (and protect the car in case of a dead short), so spec the fuse a bit lower than max current capacity of the wire (e.g. 16mm2 wire has a max amp rating of 76A, so I'd use a fuse of 70A or less to be safe).
 

Gerhard van Rooyen

Staff member
G-Wagen Club SA
Joined
Jan 21, 2009
Messages
509
Likes
37
Age
33
Location
Empangeni/Durban
Try and get this type of fuse.

https://za.rs-online.com/web/p/automotive-fuses/4316703/

Screw it directly onto the battery terminal and then screw the cable lugs into the other side. Like Paul said, both sides since any of the 2 sources can cause wiring to burn in case of a short.

I have seen similar ones at most spares shops.
Thanks Danie, that helps a lot since I was wondering how to get by the effort of splitting existing thick cables!
 

Gerhard van Rooyen

Staff member
G-Wagen Club SA
Joined
Jan 21, 2009
Messages
509
Likes
37
Age
33
Location
Empangeni/Durban
Also try and get a marine battery switch. You get very nice ones with remote operation etc.

Example: https://www.waveinn.com/nautical-fi...62lrHS8brchzRjepLTBoCXTEQAvD_BwE&gclsrc=aw.ds
Thanks Danie.
I am still learning about all of this - would this switch be between the two batteries or between the aux battery and all the devices connected to it?

I have looked it up and assume it is for the aux devices at aux battery?

https://www.wikihow.com/Attach-a-Car-Battery-Cut-off-Switch
 
Last edited:

Gerhard van Rooyen

Staff member
G-Wagen Club SA
Joined
Jan 21, 2009
Messages
509
Likes
37
Age
33
Location
Empangeni/Durban
So I borrowed an ammeter and measured the current flow to aux battery charged with 14V alternator and Cole Hersee solenoid.

100AH Lead acid battery,
10mm (78.5mm2) cables.

Battery was at 11,4V when I switched on the solenoid - charging at 12.5V (do not know why, I think the main battery was also charging) and initial load was 28.5A
this quickly came down to about 20A.
when battery reached 12V current was at around 17.5A
At 12.2V current was at 14.8A

Average current drawn while charging from 11.4V to 12.2V was 15.5A

Interesting to see.
My conclusion is that a 50A fuse is more than sufficient, also, it will protect the battery from drawing too much current for whatever reason.

Thanks for all the input guys!