Why not get the biggest battery possible?

[Dragos28]

Since we're on the topic of batteries, I've been thinking about doing a dual battery setup to run my fridge off when i'm out overlanding (not camping! lol)
But i know most batteries don't like to be drained below 50% and when i ran my fridge over night (14 hours) it drained my battery to 30% 11.97v
which was still enough to start my truck but too low and will damage my battery (odyssey)

What i'm thinking is using a wheelchair battery since they are designed to fully discharge without being damaged.

anyone else use wheelchair batteries?

 

[Pilat]

The solution to that is to get a bigger capacity battery. Wheel chair batteries are deep cycle batteries, not cranking batteries. If you calculate your needs, you should be able to find a lead acid/agm that does the job - including the fridge for 14 hours with about 50 % to spare.

Or, go get a big Braille Lithium (LiFePo4) battery if you have a lot of money to spare.

 

[Chazz Layne]

A Group 34 weighs about 55 pounds, a Group 31 weights about 70 pounds. It's a bit of a stretch to say it weights "significantly" more.


I run dual Group 31s. The stock alternator keeps up just fine. Of course, Land Rover alternators do have to put out a very happy amount of power to overcome the inherent issues with the electrical system and keep the smoke flowing.

 

[thethePete]

30% more is fairly significant.

As long as you're not exceeding your amperage output of your alternator, you won't have an issue. The problem comes when you deeply discharge the battery by powering your fridge for 3 days without starting your truck, and the alternator has to recover the battery from a deep state of discharge. If you have a 90A alternator, and you're recovering a massive marine battery, it is working at 90A (peak output) until that battery is fully recharged and it can lower its output. Alternators are not designed to charge your battery. They are designed to top it up after starting, and power your vehicle. Your battery is simply a shock absorber for the electrical system. When you start asking your alternator to charge a deeply discharged battery, it is extremely hard on the alternator. By upgrading your alternator, you're not really fixing the problem, just putting a bandaid over it by giving yourself enough overhead that the alternator recovers the system quickly enough to avoid damage. Ideally your 'house batteries' would be recharged by solar or shore power, and the alternator can simply do its job and charge the truck, or run dual alternators so the second can suffer the brunt of recharging constantly discharged batteries.

 

[AndrewP]

That isn't really true.

In an ideal world, alternators would operate at peak capacity and recharge your battery in the shortest possible time.

The reality, though, is that most batteries cannot accept a charge that fast. So a deeply discharged battery might accept 30 amps at mid charge, but for normal batteries that's about it. Remember, the charging potential is 14.0v - 12.5 volts(of the discharged battery), so the potential difference is only 1.5 volts. It takes very low internal resistance to make batteries accept a 90 amp output. Some AGM batteries may get closer to max alternator out, but normal lead acid batteries do not. Remember too that alternators put up much less amperage as they get hot. So if they out put long at max, they will get hot and the output drops significantly. Anyone had a new alternator go bad charging a dead battery?

To me, there is no down side to a bigger battery and no downside to 2 bigger batteries. More power, and particularly more STORED power is always better.

 

[toyotech]

Put the biggest one in my tundra. Love it. All other info above is to funny. Make it sound like its rocket science. I have die hard platinum in both my trucks.


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[ripperj]

Put the biggest one in my tundra. Love it. All other info above is to funny. Make it sound like its rocket science. I have die hard platinum in both my trucks.


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Umm- it's not rocket science, it's Electrical Science and Chemistry. If you care about how often you have to replace your batteries, and you care about having that reserve capacity when it's 20 below or in an emergency, then how you charge/discharge and the type of battery itself does matter.

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[toyotech]

Umm- it's not rocket science, it's Electrical Science and Chemistry. If you care about how often you have to replace your batteries, and you care about having that reserve capacity when it's 20 below or in an emergency, then how you charge/discharge and the type of battery itself does matter.

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Good thing my 5 year old platinum in my 88 runner still going strong. Stock alternator and 8k winch. Used it many times


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[Chazz Layne]

30% more is fairly significant.

22% actually, rounded up. Regardless we're talking about a mere 15 pounds, so if there's room and budget might as well go big.

 

[thethePete]

That isn't really true.

In an ideal world, alternators would operate at peak capacity and recharge your battery in the shortest possible time.

The reality, though, is that most batteries cannot accept a charge that fast. So a deeply discharged battery might accept 30 amps at mid charge, but for normal batteries that's about it. Remember, the charging potential is 14.0v - 12.5 volts(of the discharged battery), so the potential difference is only 1.5 volts. It takes very low internal resistance to make batteries accept a 90 amp output. Some AGM batteries may get closer to max alternator out, but normal lead acid batteries do not. Remember too that alternators put up much less amperage as they get hot. So if they out put long at max, they will get hot and the output drops significantly. Anyone had a new alternator go bad charging a dead battery?

To me, there is no down side to a bigger battery and no downside to 2 bigger batteries. More power, and particularly more STORED power is always better.

Except you're ignoring the part of my statement that matters, up until your admission of killing an alternator by charging a completely dead battery. I'm a licensed mechanic. I spent quite a few years learning how this stuff behaves in the real world. This isn't untested theory or Internet knowledge. I've replaced alternators and batteries just because of a bad starter. It's all interconnected and adding loads to the system shouldn't be done blindly. Like the people that tap the first switched power wire they can find without finding out what it powers or what the circuit is rated for. I've fixed some pretty sketchy stuff done by some pretty high end up fitters.

You're also ignoring the loads on the alternator just to run the vehicle. It may already be drawing 50-70A while running. Doesn't leave much head room to charge those batteries. Also lets it build a lot more self-destroying heat.

Chazz, I agree 15 lbs isn't much but between a battery a winch a bumper and everything else being thrown on the front of a rig it adds up. Usable amp hours is the key. Hence a deep cycle for house duties.

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[1Louder]

One word, Solar! I'm pretty happy with using solar instead of my dual battery setup.

 

[K2ZJ]

A Group 34 weighs about 55 pounds, a Group 31 weights about 70 pounds. It's a bit of a stretch to say it weights "significantly" more

It also differs between manufacturers. A type 65 Odyssey weighs 55lbs and a Type 65 Autocraft is only 45.

 

[Buddha.]

My approach was to install the bigger alternator and battery from the full size chevy trucks add 1/0 sized cables and hope for the best.

 

[Dragos28]

The solution to that is to get a bigger capacity battery. Wheel chair batteries are deep cycle batteries, not cranking batteries. If you calculate your needs, you should be able to find a lead acid/agm that does the job - including the fridge for 14 hours with about 50 % to spare.

Or, go get a big Braille Lithium (LiFePo4) battery if you have a lot of money to spare.

actually i am thinking about getting a lithium ion battery. I'm looking at a 40ah 12v lithium ion battery.
My only concern is that i'm not finding any information on how it needs to be charged.
Would it charge the same as a regular battery? my intention is to run a dual battery on a solenoid so it charges when the engine is running.

I'd run my fridge on it for up to 14 hours... the fridge draws at most 2ah

 

[Pilat]

It needs a higher voltage to charge, and if you use an external charger, you cannot use one that desulphates or whatever it is called that is sometimes done to lead-acids.
So, I'd check the specifics of the battery you're thinking of buying and then measure how much voltage the alternator puts out before even thinking about it I'm not so sure a mixed environment is what I'd do, even with a solenoid.