Auxiliary Battery Setup
Alright, the last pieces of the puzzle, an auxiliary power supply. This stage took the most planning. There were calculations and decisions that had to be made:
How much power do I need?
Well, it took a calculation of what does the Espar heater itself draw, what will the BCM & T-stat draw, how many hours could it potentially be running before getting an opportunity to recharge. How low of DOD (depth of discharge) am I prepared to allow the batteries to get to before recharging, the lower I am willing to accept, the shorter the life of the batteries.
Where will I mount the batteries?
Outside the truck means a substantial loss basically doubling the amount of Ah’s required at 80 deg to account for the possibility of a -30 degree night. Mounting them in the cab means better temp control, more capacity available from the same size battery but the type of battery is now restricted and will result in a more costly battery(s).
I decided on in cab using SLE batteries (Sealed Lead Acid). The best option would have been AGM batteries like a couple of Trojan AGM’s but that would have doubled the cost of the batteries which were already plenty for the SLE’s.
I calculated that I could probably get away with about 110-125 hrs of reserve if I mounted them in the heated cab so I went with 2 of these:
These are 55 Ah each wired in parallel giving me THEORETICAL capacity of 110Ah of capacity. I ball-parked it as the calculation to come up with the exact numbers is painful. Ambient temperature, rate of discharge, my determined allowable DOD, etc….suffice to say I am going to start with 2 of these and if need be, I have just enough room for one more if necessary.
Now by not going with AGM’s, even though they are sealed, there is a potential risk for gassing off with SLE’s but generally only if they are improperly charged. Whether I selected AGM’s, SLE’s, GEL's, anything but open lead acid like under the hood, the charging requirements will be different than what the alternator is delivering to the truck batteries.
Two other issues to consider is if you look at the manufacturers specs on the side of the battery in the pic above, they want to see a max initial charge rate of 16.5A, so with 2 batteries, that’s about 30Amps. So picture these batteries sitting at 50% DOD and using just an isolator or relay to separate them from the truck. When the circuit closes to charge them, they will instantly be in parallel with 1700 CCA of truck batteries… not the way I want to charge them.
So I went on the hunt for a product that would limit charge rate to about 30A, isolate them from the truck batteries and charge them properly according to SLE charging requirements.
Enter marine industry. I almost purchased a Blue Sea product (very good products) that is a very intelligent isolator but would not limit charge rate or take into account these are SLE batteries.
I then found a DC-DC charger from the UK made by Sterling Power. I bought the 30A model that limits charge rate to 30A. This product has some very key features like temperature compensation when charging, various charging profiles for different types of batteries, a maintenance program for the batteries. It really does it all.
It’s pretty cool to hook a battery charger to the truck, watch the voltage on the truck batteries climbing while the DC-DC charger is sitting dormant then all of a sudden when the truck hits 13.6V, it turns on and can deliver over 14V to the aux batteries while the truck is at 13.X volts. It then shuts back off when the truck drops to 13.3V. So regardless what voltage the truck is at, if it’s over the threshold, the charger will step up the voltage if necessary to charge the aux batteries properly.
It’s also a 4 stage charger so it will deliver a more complete charge to the batteries 5-20 times faster than the alternator can.
Here’s the unit:
Here are the two batteries sitting next to each other, the yellow thing in the green square is the temp sensor for the charger.
I did end up cutting out some rear wall liner to make room for the batteries but the good news is the cab vent is now somewhat exposed right where the batteries are mounted...that can't hurt in case of any minor venting.
I ended up moving all of the fuse blocks over more to the driver's side to make room but I managed to fit the charger and a volt meter/fuse block in:
Here it is charging the batteries; there are LED’s lit indicating input and output voltage as well as LED’s across the top indicating which stage of the charging cycle it is on.
One last thing I would like to add to this setup is the ability to turn it on from my Viper alarm remote… but that’s for a later chapter if I figure it out.
I would also like to give a shout out to Bill Darden from
Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and who helped talk me through some of the technical issues of selecting batteries, battery mounting location and charging issues. I sourced the batteries and charger on my own using the knowledge I gained from discussions with Bill.
Thank you Bill for all of your help!
If anyone wants me to post step by step detail with more pics, part #’s and source of supply in its own thread, let me know, I can probably get it done.