Battery Amps

[Bigdog]

How can I determine what my battery Amps are? I have dual batteries.

 

[Luc Lafreniere]

It's not quite as simple as you think it is. Batteries are usually rated as Ah (Amp-hours). So for example, 12V 50Ah. This means your battery can provide 50 amps for one hour. Batteries sometimes provide "cranking amps" as well. Those are the batteries used to start a motor. That is the number of amps it can provide for a short amount of time (say 10-30 seconds). But you can't sustain that for long periods.

We're talking DC voltage from a battery and therefore, batteries can provide lots of amperage unlike AC circuits like in your house. Typical home circuit provides say 15 amps AC, so that's why you can't easily use a winch since they can pull easily 30-40 amps and much higher even. In order to provide that much amperage, DC electrical is used (or converters). a 12V battery can provide tons of amperage, especially for short bursts.

So I'm not sure what you're trying to do, but based on being on a boat forum, I'm guessing you're trying to figure out how long your battery will last with a stereo or other equipment? lol

For that purpose, to get an approximate run time, you have to figure out how much load your boat will take. Let me give you my example:

The general rule of thumb I keep finding is take 10 x battery capacity in amp-hours and divide that by the load in watts. That gives you an approximate run time on the battery.

In my case, I have a single, 44 Ah battery. The load for the stereo in my case is a peak of 15 A (and after doing some research, it seems it will operate at around 10A (and 12V) because of the length of wires and such and not having it at max volume all the time). Assuming I also plug in my phone to charge, that outputs a further 2.4A (and 5V in this case). I have no idea how much amperage my dash takes, but if I had to guess, not that much so let's say 1A (and 6V? why not). That's a total load of about 10A*12V + 2.4A*5V + 1A*6V = 138W on average. So doing that math, 10x44 = 440 / 138 = 3.1 hours.

So theoretically, and based on various assumptions, my battery would last 3.1h before it's dead. Obviously not something you want to do. In the end, I installed a voltmeter that I can use to monitor the voltage of my battery as I use it. I make sure not to deplete it without running the motor for a while to charge it. I bought a cheap, USB port(s) that also provides a voltage reading. It's simple and gets the job done.

Clear as mud? Yeah, I know. Probably why nobody is answering! There is no simple answer. DC voltage is wonderful because it gives you LOTS of amperage/current, hence why they don't provide you with a current rating.

 

[Bigdog]

It's not quite as simple as you think it is. Batteries are usually rated as Ah (Amp-hours). So for example, 12V 50Ah. This means your battery can provide 50 amps for one hour. Batteries sometimes provide "cranking amps" as well. Those are the batteries used to start a motor. That is the number of amps it can provide for a short amount of time (say 10-30 seconds). But you can't sustain that for long periods.

We're talking DC voltage from a battery and therefore, batteries can provide lots of amperage unlike AC circuits like in your house. Typical home circuit provides say 15 amps AC, so that's why you can't easily use a winch since they can pull easily 30-40 amps and much higher even. In order to provide that much amperage, DC electrical is used (or converters). a 12V battery can provide tons of amperage, especially for short bursts.

So I'm not sure what you're trying to do, but based on being on a boat forum, I'm guessing you're trying to figure out how long your battery will last with a stereo or other equipment? lol

For that purpose, to get an approximate run time, you have to figure out how much load your boat will take. Let me give you my example:

The general rule of thumb I keep finding is take 10 x battery capacity in amp-hours and divide that by the load in watts. That gives you an approximate run time on the battery.

In my case, I have a single, 44 Ah battery. The load for the stereo in my case is a peak of 15 A (and after doing some research, it seems it will operate at around 10A (and 12V) because of the length of wires and such and not having it at max volume all the time). Assuming I also plug in my phone to charge, that outputs a further 2.4A (and 5V in this case). I have no idea how much amperage my dash takes, but if I had to guess, not that much so let's say 1A (and 6V? why not). That's a total load of about 10A*12V + 2.4A*5V + 1A*6V = 138W on average. So doing that math, 10x44 = 440 / 138 = 3.1 hours.

So theoretically, and based on various assumptions, my battery would last 3.1h before it's dead. Obviously not something you want to do. In the end, I installed a voltmeter that I can use to monitor the voltage of my battery as I use it. I make sure not to deplete it without running the motor for a while to charge it. I bought a cheap, USB port(s) that also provides a voltage reading. It's simple and gets the job done.

Clear as mud? Yeah, I know. Probably why nobody is answering! There is no simple answer. DC voltage is wonderful because it gives you LOTS of amperage/current, hence why they don't provide you with a current rating.

Thanks for the reply. What I am trying to do is determine my Amps so I can determine which battery charger works best for me. Is there something maybe stamped on the battery that will tell me how many amps that battery is?

 

[2kwik4u]

An Amp (or Ampere) is a measure of current draw. You batteries won't have a known current draw through them as it is directly related to what is creating that draw. Theoretically you can have a dead short (like a wrench across the terminals) and get an infinite amp draw.

I think what you are looking at is the amperage OUTPUT from the battery chargers. This will determine how "fast" the batteries are recharged.

SO, how many batteries do you have, what is their capacity, and how fast do you want to charge them?

For instance. Lets say you have a 12volt 44amp-hour battery, that is 50% charged. You need to add 22 amps to it for an hour to bring it back to full charge.....or 11 amps for 2 hours.....or 5.5amps for 4hrs......etc. Ideally you want a nice slow charge to it, so something in the 2-4amp range will probably suffice for an overnight recharge on this system. There is more to it than this, but there is the premise.

There is a guy that used to sell battery chargers in another thread. I would post your specs in there, and let him guide you.

 

[Luc Lafreniere]

To answer your question, what would be stamped on the battery is Amp-hours, Ah. Not amps because as @2kwik4u indicated, you can't get an amp rating from the battery. That number doesn't exist.

 

[Mainah]

Short answer is that bigger is usually better. More complex answer below.

Lithium batteries make ratings easier to understand as they have a max C rating for loads not exceeding a few seconds and a continuous C rating. Is this case C rating is in fact current in amps as a multiple of the capacity (ah). A 100ah 12 volt lithium battery with a max rating of 3c and a continuous rating of 0.5c is capable of a 300 amp load at 12 volts for a few seconds and 50 amp continuous load for two hours with a fairly linear relationship of load to time for anything below it continuous C rating. In most automotive or marine batteries we get cold cranking amps which would be equivalent to max C rating or max amps at its nominal voltage and a AH rating based on a defined amount of draw over time at its nominal voltage or continuous rating. Most non lithium batteries are given multiple ah ratings based on different rates as the relationship of current to time is not linear in these types of batteries. Deep cycle batteries are usually rated at a 20hr rate where you can figure out the load applied by dividing the rating by 20 and that is how much load the battery supported for 20 hours.

You can hook up a 20 amp smart charger to a motorcycle battery and not any damage as the charger will shut off the current when the voltage gets too high. Charging batteries at a high rate then tapering off to a trickle rate is actually good for them as the electrolyte needs a good shock to start accepting the electrons and then needs time to fully absorb them (Do not use smart non-lithium chargers on lithium batteries as lithium batteries do not need an absorption period and may cause them to combust. Only chargers specifically made for lithiums or to be used with their BMS should be used on lithiums). So the answer to the OP question is a bigger charger is usually better up until the charger is capable of overheating the battery before the smart circuit kicks in. A 5, 8, 10, or 20 amp smart charger will work just fine on most full size marine batteries and it all comes down to how fast you want or need to charge them. My opinion is that at least a 4 amps per bank charger with 10 amps per bank being better for boat sized deep cycle/AGM batteries should be used. Keep in mind that this is the maximum amps at which the charger will charge the battery but that it will also drop down to a lower rate for the absorption stage of charging.

 

[the MfM]

If you’re in a hurry:



If you can wait a day or two:

 

[the MfM]

*or anything in between will work.


(Don’t overthink this)