Max amp draw

Instead of trying to figure out what the MAXIMUM possible current that can be pulled from a battery, install a 200A battery post fuse.

Bobbo wrote:
To figure out the "max amp draw" that the batteries are capable of, drop a crescent wrench across the terminals. :B

A gold wedding band between the wrench and the diamond plate on the front wall works pretty well too.

For the AGM version, the Trojan data sheet shows the short circuit current as 3250A. They don't give this specification for the flooded version. In general, a flooded battery would have a higher internal resistance, and so a lower short circuit current, but I'd guess it's still in the range of thousands of amps.

The proper approach, as others have mentioned, is to size the wires for the maximum current you expect to see based on the connected load or charging circuits, and then provide appropriate overcurrent protection (a fuse or circuit breaker) sized based on the wiring. It is utterly impractical to size the wiring for the maximum current the battery can provide...except perhaps in a situation where you actually need that much current, say for a battery testing and characterization rig.

Ok so a little clarification, I have four T-105s and will be adding two more. They are within 5’ of a 600 amp buss bar, each pair is wired independently through a battery switch and to the buss bar. I plan on adding a 2800 watt Outback inverter/charger. This is being installed on my truck and will power my commercial cap for work and a truck camper for play.

theoldwizard1 wrote:
Instead of trying to figure out what the MAXIMUM possible current that can be pulled from a battery, install a 200A battery post fuse.

I guess I worded that wrong, I was trying to ask what a reasonable amp draw on a pair of T105s would be. The inverter is rated at 280amps so with two pair of T105s that would be 140 amps per pair? And I will have a fuse on each pair Of batteries

At 75 amps they are rated for 115 minutes. Of course that is dead. Closer to 45 minutes would be better.
75 x 3 = 225 amps and should be plenty to drive most anything. I assume it would be very rare to actually load the inverter to the max rating. I think you are fine with the 6 pack.

Have you considered going to 8 batteries and 24 volts?

ktmrfs wrote:

S Davis wrote:
What is a max amp draw for a pair of Trojan T105s? I am trying to figure out wire sizing.

more than likely the max current you will stuff in them to charge will be greater than the max current you pull. But you need to consider both cases. And both will depend on what you have. 50A charging is not unreasonable with a good charger, 6V jars don't like more than about 30-50A longer term draw. Disadvantage of GC is higher internal resistance which affects battery voltage on larger draw.

Charging will be a 100amp circuit from the trucks alternator and 125amp charger in the 2800 watt Outback inverter.

time2roll wrote:
At 75 amps they are rated for 115 minutes. Of course that is dead. Closer to 45 minutes would be better.
75 x 3 = 225 amps and should be plenty to drive most anything. I assume it would be very rare to actually load the inverter to the max rating. I think you are fine with the 6 pack.

Have you considered going to 8 batteries and 24 volts?

I will eventually have 8 batteries in the system, six on my truck and two in my work cap and truck camper. I want to use the truck for charging so going 24 volt would be an issue.

How well ventilated is your battery compartment, if one exists? In high draw conditions, wet cell (lead/acid) batteries will eventually off gas hydrogen and oxygen in large enough quantities to go boom. (not good)

Chum lee

Chum lee wrote:
How well ventilated is your battery compartment, if one exists? In high draw conditions, wet cell (lead/acid) batteries will eventually off gas hydrogen and oxygen in large enough quantities to go boom. (not good)

Chum lee

Yes that can be an issue, I worked on a off grid project 12,000sqft house on a 40 acre island in the San Juan islands where the they put one of the main battery boxes in the basement and just about launched the house.


The four are in vented battery boxes in the front of the truck bed, the other two will be mounted to the truck frame rails with custom brackets. The two in the camper will be in factory battery box and the two in my commercial cap will be in a sealed vented box.