Lithium battery Question

Red Rover

Red Rover

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Joined
Jun 26, 2015
Location
Wilmington
If the purpose of the alternator is to run the loads on the vehicle then why do we keep going with larger batteries? Let's say I have an appropriately sized upgraded alternator for my stock jeep loads plus extras like a winch, lights, fans, etc. Then why would I need for example a 1000 cold cranking amp battery? Don't I only need a battery large enough to start my engine? Then allow the alternator to do the work. If I am not mistaken I believe the 4.0L starter only draws 160-200amps on startup depending on the how new the engine is, ambient temperature, etc and a fuel pump is only 20A at max. My question is, can I get away with a 300-400 CCA lithium battery and still be able to run my extras? Its lighter, smaller, and will fit better when I decide to pinch the front end without having to relocate my giant, 900cca deep cycle AGM battery...lol
 
The purpose of the alternator isn't to run the vehicle loads. The purpose of the alternator is to keep the battery charged. The battery has the reserve capacity to handle a large amount of loads starting at once or turning on and off multiple times. We keep getting larger batteries or more powerful batteries because we load the vehicles with more and more electrical loads.
 
Stuntman Autoworks said:
The purpose of the alternator isn't to run the vehicle loads. The purpose of the alternator is to keep the battery charged. The battery has the reserve capacity to handle a large amount of loads starting at once or turning on and off multiple times. We keep getting larger batteries or more powerful batteries because we load the vehicles with more and more electrical loads.
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Yes, the purpose of the alternator is to run the vehicle loads and charge the battery. You can't separate those two functions. The battery and alternator both supply vehicle loads.

Larger batteries are only a benefit if you're going to be using a lot more battery power versus time than the alternator can recharge, which either means that you're using devices with the engine off, or you've failed to properly size your alternator so you're running in constant depletion mode . Thats what reserve capacity is.

Otherwise people get a bigger battery because it has more parallel internal plate area, which lowers the internal resistance. That's where higher CCA rating comes from for a given type of battery (make it bigger, more CCA). That lower internal resistance is what helps with big electrical loads turning on and off, not the reserve capacity. For example a big deep cycle battery will have big reserve capacity, but relatively poor internal resistance, which makes it a bad choice for big transient loads because of voltage drop.

So yes, with a properly sized alternator, you could downsize the battery. But you're decreasing your safety margin for certain conditions, and you may see higher voltage drop with a smaller battery under certain conditions (maybe running a winch for example).

If you want to throw money at it, a racing battery that has low internal resistance can supply big cranking amps in a very small package, but will have small reserve capacity. You could also run two 6V Optima batteries in series, and put the batteries in different locations (does Optima still make those 6volt batteries?).
 
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Think of it like an air compressor. The tank supply’s the instant demand you need in the quantity needed while the compressor keeps adding to the tank and maintaining what you need. The compressor alone can’t supply what’s needed, nor can the tank. But the two working together can.

Alternator+battery does the same thing.

Also, a large deep cycle battery is good in a situation where you might not have a running Jeep, but you still need to run your winch and a couple lights.
 
Another view point. A battery starts dieing the day you place it in service. It loses cranking ability with every operation. Now the loss is miniscule, but its still losing.
In contrast as electric motors age things like internal resistance cause the required demand to increase with time. You have 2 trains running down the same track towards each other.
If you start close to limit you will be replacing sooner than if you start with a big reserve that dwindles towards minimum acceptable.
 
77GreenMachine said:
Also, a large deep cycle battery is good in a situation where you might not have a running Jeep, but you still need to run your winch and a couple lights.
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That's not what deep cycles are made for. Suck 600A out of a deep cycle at once, and you'll kill it. Deep cycles are for campers, golf carts, and trolling motors.

Stuntman Autoworks said:
The purpose of the alternator isn't to run the vehicle loads. The purpose of the alternator is to keep the battery charged.
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If the alternator isn't covering the vehicle loads, either the alternator light is going to come on (old cars) or the ECM is going to decide that the alternator is crapping out and kill the field voltage to it (new cars).

I used to run one of the small Odyssey batteries. It was like 500 CCA. Under normal circumstances, it would start the vehicle just fine. But if you let it sit for a week, or ran the headlights without the engine running, etc, it would draw down to the point where it might not start.

Considering we often need to run the winches when the engine isn't running, or want to run lights or radios without the engine on, I get the biggest fawking battery I can fit (and that I'm willing to pay for). I find they also tend to last a bit longer. They still have enough ass in them once they get older to do the job, even though the actual capacity is less than what it was when they were new.
 
I had two 1000amp batteries go dead when I broke once in the middle of a very tough trail. Tell me how many amps alternator would I have needed without using the battery power.
 
GubNi said:
I had two 1000amp batteries go dead when I broke once in the middle of a very tough trail. Tell me how many amps alternator would I have needed without using the battery power.
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Going to need a lot more information than that to answer the question....
 
In my wheeling group we've self recovered a few times (on our side or upside down) and used our own winch to roll back on the wheels, can't do that on alternator if the engine is off.

If the rig is stuck stuck it's not impossible to see 400 or more amps being drawn by the winch. If you over exceed the capacity of an alternator the voltage drops, and most efi will shut down to protect the ecm if voltage drops into the 10v range roughly. We spent a miserable 3 hours at the entrance to mason jar in Harlan with a friends truck, his battery had shorted internally and it was running on just the 130 amp alternator. It was pouring rain and slick as snot and every time he'd pull the slack out with the winch it would stall. Finally had to swap batteries with another rig so he could get up the hill.

It's much more efficient both in space and mpg to have an oversized battery for reserve capacity and short term high amp draw, and a moderately sized alternator that can handle normal loads (fan, AC, lights, fuel and spark) with enough reserve to be able to top the battery off.

I've seen 3 and 400 amp alternators on bucket trucks for the phone company and they are twice the diameter of a regular 100 to 130 amp alternator. The more amps an alternator can push the more heat it produces so it needs bigger fans heatsinks and windings.

Sent from my HTC6545LVW using Tapatalk
 
shawn said:
If the alternator isn't covering the vehicle loads, either the alternator light is going to come on (old cars) or the ECM is going to decide that the alternator is crapping out and kill the field voltage to it (new cars).
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By keeping the battery charged the alternator is in a sense covering the vehicle loads. But it is not designed to be the only source. It's not even designed to be a "battery charger" like you would have in your shop. It's really more of a maintainer.
 
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