Dual battery questions

[hoosierboy]

I know there is a dual battery thread but rather than showing what I'm building I'd like to ask questions here before begin inveating in equipment.
I'm not interested in brand name or vehicle specific applications. I want to build a reliable system that can meet current and (more importantly) future needs.

Topics
1. Battery type: I'm thinking of running a bank of smaller (lawn mower sized) lithium batteries. This will be beneficial to space and battery performance, at least that's the hope. Are there any special considerations for litium batteries?

2. Charging: I know I will need a battery isolator. Recommended style/type?

2a: Should I consider adding a second alternator or high output alternator? My end game is a full self sustained rig with fridge, shower, all of the lights, multiple radios, gps units, many devices needing charged, probably an inverter, etc. Eventually I will add solar to help charge but it would be nice to end the day with fully charged batteries to get me theough the night.

3. Setup: Obviously feild repairs are inevitable. Any advice around designing an easy to diagnose system would be greatly appreciated.

4. Capacity: For a fully sustained rig, what should I shoot for? I will likely build to be expanded later but I want to plan ahead for additional batteries/battery banks.

Please feel free to spill all of your 12VDC brain juice in this thread. Thanks in advance!

 

[Plasmajab]

Of course this will all depend on your plan and current set up. For this Im just going to assume you currently have a single battery set up, Lead/acid.

1. Lithium batteries are good, but they do not handle temperature differences well. In a toasty engine bay they will have a shorter lifespan and are only good for so many charge/discharge cycles (like all batteries) before they inevitably fail. There are also different boosting procedures required and as well, different runaway fighting procedures depending on the chemistry. I suggest using an AGM style battery as they do not spill should you end up at unfavorable angles.

2. Battery isolators are a great investment. What I recommend is to use two simple battery master switches, something simple like this. https://www.princessauto.com/en/detail/keyed-battery-master-cut-off-switch/A-p8105033e

Run your batteries in parallel. From the factory battery run 2 lines to the Auxiliary battery and put your switches in between. Run your "long term" batteries off the Auxiliary battery (scene lights, fridge, water heater). When you drive around leave the switches open so that the auxiliary battery charges. When you camp, switch them off so that the draining accessories are pulling only from the auxiliary. That way even if you drain that battery to zero, your factory battery will still be able to start the rig, then you can flip the switch to charge the drained auxiliary battery.

2a. When running a dual-battery system I definitely recommend switching to a high output alternator or a secondary.

3. KISS. What I recommend is from the auxiliary battery, run 2 heavy gauge wires to the back and set up two distribution blocks you can clip your accessories on with ring terminals so they dont bounce loose on the trail. Use proper fuses as close to the battery as possible.

4. What I would recommend is calculate the amount of amps your rig will use when running everything at the same time. For example. Lets say your'e pulling 20 amps. Look at the spec of the battery you are looking for. Ill use optima batteries for ease of explantion.

OPTIMA Redtop 34
38.8lbs,
CR20 Capacity 50AH
CCA 800A

OPTIMA Yellowtop D34
43.5 LB
CR20 Capacity 55AH
CCA 750A

OPTIMA Bluetop D34M
43.5LB
CR20 Capacity 55AH
CCA 750A

What's important here is the CR20 Capacity. This is the measure of how much energy the battery can provide over 20 hours without damaging the battery. So if you bought a redtop 34, you can safely pull up to 50amps an hour over 20 hours without damaging the battery or discharging it to flat.

Granted of course, you aren't going to use 50 amps out there. But say your average use will be 10 amps to run your fridge and some lights and the odd radio. Perfect world would tell you at a ten amp draw would run roughly 4-5 days. But of course there is some variance. My suggestion is before you do your install, charge up the auxiliary battery on your garage floor with all your stuff that is going to run and let it go, see how long it will take. This will also give you familiarity and the opportunity to fault find off the vehicle before you install and make it easier to fix out in the woods.

Some batteries do not have the C1,c3,c5,c20 rating system and will only show the Amp hours (AH) on the label. If the battery has 500AH written on it, then it can provide 1 amp of power for 500 hours. 10 amps for 50 hours, 20 amps for 25 hours, etc...

 

[hoosierboy]

Excellent information! Thank you so much. That gives me a great starting point. Just one question, Is their some kind of high amp relay I could use in place of the manual battery switch? It would be nice if I could find a turn-on wire off the ignition or something that would connect when on and disconnect when the motor is off.

 

[Plasmajab]

Absolutely. I have seen people use old ford solenoids wired into the ignition hot wire. So when the ignition goes hot, both batteries are linked. When off, batteries are separated.

Not sure about relays but I imagine something similar would work just fine.

 

[hoosierboy]

I just used the term "relay" because that's the closest thing I could think of. Maybe what I'm looking for is a solenoid.

I can't think you enough. I'm already building a shopping list. It might be a while before I can get it all put together as I am just starting out but this gives me something to start with and I can always upgrade as needed if I find a weak link in the system (i.e. not enough AH).

 

[Plasmajab]

Relay, solenoid, same function really. I think you can get 50-60A relays these days. Id start with calculating your needs, then leave a buffer for upgrades then watch the visa card cower in fear!

 

[hoosierboy]

For sure! I can already feel it shaking in my wallet.

I will definitely calculate needs before purchasing. At least now I have a good idea of what to look for. Thanks again.

 

[v_man]

What helped me most when wrapping my head around a dual battery install was reading other folks write ups on their install. Seeing pictures, products used, the end result, was very helpful to me. Here is my write up:

https://www.overlandbound.com/forums/threads/dueling-batteries-a-dual-setup-on-my-07-lx.9245/

My dual setup involves a completely manual, user operated, solenoid to connect and disconnect my two batteries. You need to research and decide if you always want your two batteries automatically connected when car is running, and disconnected when car is off.

For our needs, it's preferable to have total control over battery connection regardless of wether the car is on or off. I can think of scenarios when it would be best to have complete autonomous control of how your batteries are linked or connected....

 

[Baipin]

Absolutely. I have seen people use old ford solenoids wired into the ignition hot wire. So when the ignition goes hot, both batteries are linked. When off, batteries are separated.

Not sure about relays but I imagine something similar would work just fine.

This is similar to what I did for the dual battery setup in my Forester; tapped an empty fuse location which would only get power when the engine was running. This, in turn, would close the solenoid contacts and let power flow from the alternator to the second battery. When the engine is off, the contacts open and prevent the starting battery from being drained. I keep the second battery - a 36Ah AGM - in an ammo case with venting, Anderson plugs, and a couple 12V receptacles. Very simple, very cheap, very effective!

In the future, I'm thinking of adding a manual override for the solenoid, which could possible help with cranking in cold, -40 weather.

 

[Plasmajab]

Not a bad idea. As a Canadian I can attest to some weird cold weather effects. It would seem that in addition to dual batteries, the car can sometimes be frightened to run if you scream it it loud enough. 'Start'led if you will.

 

[hoosierboy]

v-man:
Excellent write-up.Thanks for sharing. I see your point about the manual control. I think I will do a hybrid. 12v turn-on, to switch, to solenoid. That will ensure I can always disconnect manually but will also ensure the solenoid is disengaged when the vehicle is off. I can guard against draining the start batt and make sure start batt gets a good recovery charge.

Baipin:

That seems like a great and cheap setup. I think I will require a little more than 36AH but I will calculate that later. Love the simplicity though. Cost is of course a concern of mine as well so I will be shooting for something similar just more AHs.

Plasma:

Believe it or not, here in Indiana I have experienced the same thing. It's rare, (once every 5 years or so) but it does happen.


You guys are awesome. This is my first thread on OB and so far I am thoroughly impressed with the helpful advice. Thank you!

 

[JPaul]

Of course this will all depend on your plan and current set up. For this Im just going to assume you currently have a single battery set up, Lead/acid.

1. Lithium batteries are good, but they do not handle temperature differences well. In a toasty engine bay they will have a shorter lifespan and are only good for so many charge/discharge cycles (like all batteries) before they inevitably fail. There are also different boosting procedures required and as well, different runaway fighting procedures depending on the chemistry. I suggest using an AGM style battery as they do not spill should you end up at unfavorable angles.

2. Battery isolators are a great investment. What I recommend is to use two simple battery master switches, something simple like this. https://www.princessauto.com/en/detail/keyed-battery-master-cut-off-switch/A-p8105033e

Run your batteries in parallel. From the factory battery run 2 lines to the Auxiliary battery and put your switches in between. Run your "long term" batteries off the Auxiliary battery (scene lights, fridge, water heater). When you drive around leave the switches open so that the auxiliary battery charges. When you camp, switch them off so that the draining accessories are pulling only from the auxiliary. That way even if you drain that battery to zero, your factory battery will still be able to start the rig, then you can flip the switch to charge the drained auxiliary battery.

2a. When running a dual-battery system I definitely recommend switching to a high output alternator or a secondary.

3. KISS. What I recommend is from the auxiliary battery, run 2 heavy gauge wires to the back and set up two distribution blocks you can clip your accessories on with ring terminals so they dont bounce loose on the trail. Use proper fuses as close to the battery as possible.

4. What I would recommend is calculate the amount of amps your rig will use when running everything at the same time. For example. Lets say your'e pulling 20 amps. Look at the spec of the battery you are looking for. Ill use optima batteries for ease of explantion.

OPTIMA Redtop 34
38.8lbs,
CR20 Capacity 50AH
CCA 800A

OPTIMA Yellowtop D34
43.5 LB
CR20 Capacity 55AH
CCA 750A

OPTIMA Bluetop D34M
43.5LB
CR20 Capacity 55AH
CCA 750A

What's important here is the CR20 Capacity. This is the measure of how much energy the battery can provide over 20 hours without damaging the battery. So if you bought a redtop 34, you can safely pull up to 50amps an hour over 20 hours without damaging the battery or discharging it to flat.

Granted of course, you aren't going to use 50 amps out there. But say your average use will be 10 amps to run your fridge and some lights and the odd radio. Perfect world would tell you at a ten amp draw would run roughly 4-5 days. But of course there is some variance. My suggestion is before you do your install, charge up the auxiliary battery on your garage floor with all your stuff that is going to run and let it go, see how long it will take. This will also give you familiarity and the opportunity to fault find off the vehicle before you install and make it easier to fix out in the woods.

Some batteries do not have the C1,c3,c5,c20 rating system and will only show the Amp hours (AH) on the label. If the battery has 500AH written on it, then it can provide 1 amp of power for 500 hours. 10 amps for 50 hours, 20 amps for 25 hours, etc...

I don't know where you came up with this CR20 naming from, I have never heard of that particular nomenclature and can't find it in Google. I think you just mean what the 20 hour rate is, just as there are other multiple hour ratings (10hr, 100hr, etc). Batteries don't actually supply their rated AH no matter how long or great the draw is. A higher draw on the battery will result in fewer total AH being available, while a lower draw will eke out more AH of capacity.

But it certainly does not mean the group 34 rated at 50AH is going to be able to sustain 50 amps an hour for 20 hours without going flat. It means it can discharge up to 50 amp hours total over a 20 hour period. So that really means that after 20 hours of being discharged at a rate of 2.5AH it will be considered completely discharged. 50AH over 20 hours would be 1000AH.

If you draw 5AH then you'll probably end up with a useable capacity of more like 48AH or so, meaning you can't even simply divide or multiply your capacity by the draw. 5AH draw will flatten the battery in less than 10 hours, and a draw of 50 amps can flatten it in far less than an hour. You'll need a chart from the manufacturer for that particular battery to determine what the different discharge rates and corresponding capacities are. It is not linear though so keep that in mind as you design your system.

 

[Baipin]

Another thing worth noting; 48 Ah seems to be the minimum. I'm struggling a bit on 35Ah even just charging a laptop and flashlight (neither of which were completely drained). A smaller battery + an Optima Yellowtop is something to consider is space is at a premium though. (Don't neglect your ability to reliably crank the engine though)!