Dual Battery Strategy

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TeCKis300

TeCKis300

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I recently went out on an camping trip with the family where I had my battery go dead. It was just a slight inconvenience in this situation, but I can certainly imagine a trip where the repercussions are far more serious. It's got me considering a dual battery setup.

So I've been exploring the dual battery configs out there and as far as I can tell, most everyone goes with two like batteries, in a dual bank configuration. One as a starter battery and one as a 'house' battery.

While I can definitely see the merits of this configuration, a limitation is that the total capacity for each of the functions that each bank serves, is limited to half the total combined capacity. For example, the fridge or aux lights can only access 50% of the total battery capacity before running out. Or car stereo and interior fans can again only access 50% before running out.

Why then, would one not want to setup the batteries in a primary / backup configuration. Where the primary batt and backup batt has an 80%/20% capacity split. For example, I'd get a group 31 or 65 primary, and a motorcycle type battery for the backup starter battery. Is there a tragic flaw in this that I'm not seeing? This would allow whatever function of my car that I'm using, to have full access to more reserve capacity.

Couple additional bonuses is that I'm less likely to deep discharge any particular bank. And I can find a place in my AHC LX to accept a smaller battery underhood.

Thoughts?
 
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I've had dual batteries before on a previous expo rig, albeit, I had that installed for me and didn't do the work myself, but my was a "starter" and "house" setup. So I think the short answer to your question is that the dual battery kits out there are set up as a "starter" and "house" battery arrangement.
I've never looked into this before, so I'm not sure if there are any kits that do it like that, or how to wire it up.
I'm a mechanical engineer, my electrical knowledge is limited to red and black don't touch.

Interesting idea though, and would help with weight and space under the hood.
 
Most systems have the feature that allows you to tie the batteries together, so if the starter battery is dead, the backup battery can start the vehicle

I have gone 3 days with the backup battery powering an ARB Fridge. If the battery ran out, I would start up the vehicle to recharge
 
O.K.
First of all I would not want to rely on a "motorcycle" size battery as my only way to start my V-8, even in the best of conditions. Not only that, if you did have that set-up. as soon as you flipped the switch to connect the two batteries together to run them in parallel to jump your primary battery......Bammm. That dead or near dead battery would suck the life right out of that motorcyle battery and then you would really be stranded.
 
Not necessarily
 
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Romer said:
Most systems have the feature that allows you to tie the batteries together, so if the starter battery is dead, the backup battery can start the vehicle
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Right, definitely would want a similar thing for this setup.

badlander said:
O.K.
First of all I would not want to rely on a "motorcycle" size battery as my only way to start my V-8, even in the best of conditions. Not only that, if you did have that set-up. as soon as you flipped the switch to connect the two batteries together to run them in parallel to jump your primary battery......Bammm. That dead or near dead battery would suck the life right out of that motorcyle battery and then you would really be stranded.
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Motorcycle batt might be a bad description. Let's just say a relatively smaller battery. Maybe one or a couple sizes down from the stock batt. Something that would start the car, but didn't necessarily have to have significant reserves.

Are you suggesting with very dissimilar sizes and state of charge, that I would want a switch that can completely isolate the primary large battery while I use the smaller to start the car? Doable and something I'll look into.
 
A normal setup is the main starter battery, as the one you have, plus an aux battery which feeds all the aux equipment.
In my previous car Ihad that setup and it worked perfectly.
The fridge was runningfrom the aux battery while the aux battery was charged when driving.
Remember 50% of 2 batteries is more than 50% of 1 battery.
 
If you haven't already seen this thread, take a look at Rohitash's set-up. He is using a PC925 which is a big motorcycle battery.

As far as using different battery sizes, I'm curious about this too. Most battery and isolator companies will tell you to use the exact same battery (same age too). But many people have used different batteries, even mixing lead acid with AGMs with an isolator, for years w/o problems.... so :meh:. In any case, I think every now and then, it's a good idea to charge your AGM batteries with an AGM compatible charger.
 
Wilsil said:
A normal setup is the main starter battery, as the one you have, plus an aux battery which feeds all the aux equipment.
In my previous car Ihad that setup and it worked perfectly.
The fridge was runningfrom the aux battery while the aux battery was charged when driving.
Remember 50% of 2 batteries is more than 50% of 1 battery.
Click to expand...

I understand the aux battery, or house battery, serving the auxiliary equipment like external lights, fridge, etc. But I oftentimes use the starter battery too while the car is off, for things such as interior lights, cig lighter chargers, etc. So with two equal sized banks, I have to manage the use of each, and remember not to run at least one bank down too far so I can start the car.

With one large primary batt and small secondary, I don't have to manage the divided use of each bank. Just use away and I always have the small backup batt in case. I'd imagine this would serve better for parasitic situations like a fridge as it's much less likely to deep cycle a relatively larger primary batt.

Of course I can always bridge the two separate banks in the first config to access full capacity, but I do have to be a bit more careful in that scenario not to empty the combined banks.


Sandroad said:
There are a few things/ideas to play with on this site. There are lots of different demands for dual (and multiple) battery set-ups in boats.

Electrical - Batteries & Accessories at West Marine
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Thanks for the link, I'll look into it.

hoser said:
If you haven't already seen this thread, take a look at Rohitash's set-up. He is using a PC925 which is a big motorcycle battery.

As far as using different battery sizes, I'm curious about this too. Most battery and isolator companies will tell you to use the exact same battery (same age too). But many people have used different batteries, even mixing lead acid with AGMs with an isolator, for years w/o problems.... so :meh:. In any case, I think every now and then, it's a good idea to charge your AGM batteries with an AGM compatible charger.
Click to expand...

Ah, I like where he mounted the second battery. I've had a PC925 in my turbo Lexus IS300 for years which is kinda the size I'm thinking about.

I'm now toying with the idea of a group 65 primary and group 58 secondary, which I can get from Costco for $75 and $54 respectively might be the ticket. Toss in a descent battery isolator relay for $30 and I've got the capacity and insurance I'm looking for.
 
I think your plan could work, sans the motorcycle battery.

If you hooked your batteries up like mine (https://forum.ih8mud.com/100-series-cruisers/607791-t-max-dual-battery-install.html), you could run two different batteries because the 2 batteries never see each other except during charging. When draining, they automatically separate.

To keep your 2nd battery as a back up, you could just run all your accessories off of the starter battery and keep the aux battery just for back up starting via the link function on the IBS/T-Max system. That's how mine's set up now because I haven't gotten around to wiring up accessories.
 
KlausVanWinkle said:
If you hooked your batteries up like mine (https://forum.ih8mud.com/100-series-cruisers/607791-t-max-dual-battery-install.html), you could run two different batteries because the 2 batteries never see each other except during charging.
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Well, that's exactly the argument, while charging, different types batteries have different charge rates and charge requirements. One of the two batteries may never get fully recharged. You may be only getting 85% of the rated capacity.

My understanding is batteries in general liked to be fully charged and only partially discharged for longevity sake. i.e. 100% charged then discharged to 85% and then charged again.

But I really don't know enough about batteries and charging batteries to form an opinion.
 
One thing to keep in mind that most people totally overlook. The alternator is a VERY poor charger for batteries. It's main job is to generate power for everything onboard while the engine is running. As a by product is provides a trickle charge to the battery, so it's hopefully topped off next time you go to start the vehicle.

For this reason I will be mounting in the engine bay a pair of CTEK chargers that will be permanently wired to each battery (main and aux). I will then regularly plug AC power into those chargers when parked in the driveway to make sure each battery gets a proper 100% charge. Especially coming back from a long trip where the batteries saw high usage and the alternator is almost certainly struggling to bring the batteries back to 100%.
 
I wouldn’t hesitate to mix battery types in such a configuration if the secondary batt is used only as a backup. But it does take away the ability to bridge the batteries to function as a common bank because AGM batteries operate at a slightly different voltage level than traditional batts. With two batts, I wouldn’t pay the AGM premium anyhow as each works as a backup to the other, meaning neither is as critical. For the cost premium of an AGM, I could replace them at least 3x over with a standard batt. And the AGM is not going to last even 2x as long for my use.

Robred, I did think of a portable solution, but an installed solution seems to be the better value. I like the idea of a secondary battery being there to cover things like an alternator failure, extra reserve, or extra current handling. Besides, those things seem like toys that I’d only rely on for inconvenience situations in the city.
 
Anthony.L said:
One thing to keep in mind that most people totally overlook. The alternator is a VERY poor charger for batteries. It's main job is to generate power for everything onboard while the engine is running. As a by product is provides a trickle charge to the battery, so it's hopefully topped off next time you go to start the vehicle.
......
Click to expand...

I disagree with this, the alternator is a very good charger. The alternator in a car is designed to run the systems and charge the battery. Anything less would be very poor design indeed. Consider that even for trips of very short duration the battery is maintained at full charge, think about the last time you ran errands around town. Your car started every time assuming the battery and charging system were in proper working order.

Where people run into problems is when they modify the original system. ie add lights, winches, auxiliary batteries etc.
 
What Rusty said: an alternator is about the best charger there is for your car battery. It is purpose-designed for that use, has a nice regulator, is 100% duty cycle rated even while heat soaking in an engine bay, and puts out 90-130. Whoever told you that an alternator 'only trickle charges' the battery is wrong, full stop.
 
While I do agree with rusty_tlc and jOnathN, do note that the output of the alternator is rpm dependent. So at idle, it won't charge the batteries at the same rate as if you were running on the highway. And if you are seriously loaded with electric parasites (stereo, lighting, fridge, etc.) and you have a weak or undersized alternator, you might actually have a net deficit of current from the alternator at idle and still be drawing charge from the batts. Unlikely for most of our setups, but a possibility.
 
jOnathN said:
What Rusty said: an alternator is about the best charger there is for your car battery. It is purpose-designed for that use, has a nice regulator, is 100% duty cycle rated even while heat soaking in an engine bay, and puts out 90-130. Whoever told you that an alternator 'only trickle charges' the battery is wrong, full stop.
Click to expand...

So you are saying Toyota had the forethought to design their alternators to be optimized for charging AGM and other gel cell battery technologies? Guess all the companies out there selling newer technology chargers for just that purpose are blowing smoke up everyone's ass?
 
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