Trickle Charge Battery For Power while Drawing on it?

[warewolff]

Originally Posted by superdave

well when my fridge is on 3.1 amps ac it draws 3.1 amps dc on my 24 volt system. that's just reality. try again

No, it doesn't. If your fridge is pulling 3.1 amps at 120vac, you're pulling 372w. 3.1 amps x 120v = 372 watts. Watts is constant. If you're pulling 372w and using an inverter to pull from your 24v system, you're pulling ~15.5 amps from your batteries. 24v x 15.5 amps = 372 watts. It's not disputable. That's reality. If you were to measure the draw on your batteries when your fridge is running, you'd find it pulling 15.5 amps. Probably a little more because of efficiency losses at the inverter. Just because you read 3.1 amps at your outlet with a Kill-a-What meter doesn't mean you're pulling 3.1 amps at the batteries. I assume you are using an inverter to run that ac fridge, and if that's the case your inverter pulls 15.5 amps at 24v. That HAS to be the case, because your 120v ac fridge won't run on 24v batteries. Period. Amps x Voltage = Watts. You've had it explained to you now multiple times by informed members. I work with electricity. I am explaining to you how it works. If you have additional questions and a desire to learn, I am happy to respond again.

[Twigg]

You really don't know when to quit.

Here is Ohm's Law:

P = V x I

where P = Power, V = Volts, I = Amps

So let's do some sums.

Let's assume P = 1000 Watts @ 120 Volts

From Ohm's law we get 1000/ 120 = 8.33 Amps

Now let's shift the voltage to 12V

1000/12 = 83.33 Amps

These are laws of physics. I didn't make this up, a really smart guy called Georg Ohm figured it out. Well he actually figured out that current = voltage over resistance, but a bit of extra math came up with the formula for power.

At this point I am about to recommend you farm out electrical work.

[warewolff]

Originally Posted by Twigg

At this point I am about to recommend you farm out electrical work.

Agreed. He's going to end up running a vastly undersized wire "because it's only 3 amps!" and burn his bus down. You can lead a horse to water ...

[BigSteveATX]

Thanks everyone for input so far! So, to be clear, I'm installing a car stereo, not a home audio system. The batteries are going to be the AGM type like this: https://www.sonicelectronix.com/item...N-SK-BT45.html

From what I've read, I think that the 110 to 12v inverter is a good option. Something like this: https://www.amazon.com/Powermax-Supp.../dp/B00F8MC42C It seems that this is built to charge at the same time as powering the 12 volt stereo/amps/subs/etc. What I'm not certain of is the amount of AMPs I need. It looks like there's 35, 45, 55, amp systems and on up to 75 amps. My guess is just to buy the bigger one and see how it plays out.

Also, I don't want to have the generator on all the time for the stereo to run, that's why I'm buying the batteries to help the system while the generator is off.

[BigSteveATX]

I think that's the way I'll go. There will be a battery (actually 2) to run the stereo while the generator is not on.

[warewolff]

Originally Posted by BigSteveATX

I think that's the way I'll go. There will be a battery (actually 2) to run the stereo while the generator is not on.

Sorry to threadjack. That misinformation is dangerous though. What you're looking for is an inverter/charger and a battery backup. You can plug in to shore power or run your generator and charge everything up, then drive away and drain from your batteries seamlessly. I have an expensive Magnum model but there are cheaper alternatives (Xantrex is quite good from what I understand.) Just make sure your battery backup is large enough to support the stereo system while you're driving. If you're pulling 1.5kW per hour and want to drive around for an hour off of a separate set of batteries, you'll need at least a 3kW battery backup. (Even with deep cell lead acid batteries you generally don't want to discharge past the 50% mark). I think you might also find that upgrading your alternator and running a line to charge the batteries from there might be another option. That way you can charge up while you're stationary with the inverter/charger, drive and flip a switch to charge with your alternator, stop and flip the switch off, back to shore power or generator. Does that make sense? I can point you in the right direction as to specific hardware if you like.

[Twigg]

Originally Posted by warewolff

Sorry to threadjack. That misinformation is dangerous though. What you're looking for is an inverter/charger and a battery backup. You can plug in to shore power and charge everything up, then drive away and drain from your batteries seamlessly. I have an expensive Magnum model but there are cheaper alternatives (Xantrex is quite good from what I understand.) Just make sure your battery backup is large enough to support the stereo system while you're driving. If you're pulling 1.5kW per hour and want to drive around for an hour off of a separate set of batteries, you'll need at least a 3kW battery backup. (Even with deep cell lead acid batteries you generally don't want to discharge past the 50% mark). I think you might also find that upgrading your alternator and running a line to charge the batteries from there might be another option. That way you can charge up while you're stationary with the inverter/charger, drive and flip a switch to charge with your alternator, stop and flip the switch off, back to shore power. Does that make sense? I can point you in the right direction as to specific hardware if you like.

This ^^^

Except he isn't pulling 1.5kW per hour, nor even close. The amplifier power rating is not the power consumption.

OP could get a better idea by plugging the system into a Kill-A-Watt and running it for an hour at the normal volume. That would give an idea of the battery size needed and an inverter might not be necessary.

It depends on the power draw.

[warewolff]

Originally Posted by Twigg

This ^^^

Except he isn't pulling 1.5kW per hour, nor even close. The amplifier power rating is not the power consumption.

OP could get a better idea by plugging the system into a Kill-A-Watt and running it for an hour at the normal volume. That would give an idea of the battery size needed and an inverter might not be necessary.

It depends on the power draw.

I don't know exactly what his system will draw but was giving him an example. I'd definitely err on the side of oversizing the battery bank though, as while you don't want to draw past 50%, you ALSO don't want to pull more than C/8 or C/10 (12.5 to 15% of the battery's total capacity) at once. So if I wanted to run a 1.5kW load, I should really have a 10kW battery backup. I do on my own bus . Anything more than that leads to sulfation, and decreased battery longevity. Even running his batteries down to 50% that often will cause a lot of sulfation. He'll have to run an equalization on a biweekly basis.

[superdave]

Originally Posted by warewolff

No, it doesn't. If your fridge is pulling 3.1 amps at 120vac, you're pulling 372w. 3.1 amps x 120v = 372 watts. Watts is constant. If you're pulling 372w and using an inverter to pull from your 24v system, you're pulling ~15.5 amps from your batteries. 24v x 15.5 amps = 372 watts. It's not disputable. That's reality. If you were to measure the draw on your batteries when your fridge is running, you'd find it pulling 15.5 amps. Probably a little more because of efficiency losses at the inverter. Just because you read 3.1 amps at your outlet with a Kill-a-What meter doesn't mean you're pulling 3.1 amps at the batteries. I assume you are using an inverter to run that ac fridge, and if that's the case your inverter pulls 15.5 amps at 24v. That HAS to be the case, because your 120v ac fridge won't run on 24v batteries. Period. Amps x Voltage = Watts. You've had it explained to you now multiple times by informed members. I work with electricity. I am explaining to you how it works. If you have additional questions and a desire to learn, I am happy to respond again.

yes it does sir, you are wrong. amps are amps no matter the voltage. come on over and i will show you. my bogart will not lie

[superdave]

Originally Posted by warewolff

I don't know exactly what his system will draw but was giving him an example. I'd definitely err on the side of oversizing the battery bank though, as while you don't want to draw past 50%, you ALSO don't want to pull more than C/8 or C/10 (12.5 to 15% of the battery's total capacity) at once. So if I wanted to run a 1.5kW load, I should really have a 10kW battery backup. I do on my own bus . Anything more than that leads to sulfation, and decreased battery longevity. Even running his batteries down to 50% that often will cause a lot of sulfation. He'll have to run an equalization on a biweekly basis.

for maximum battery life you do not want to drop your soc past 80 percent. anything past 60 will damage the batteries.

[jazty]

Originally Posted by superdave

yes it does sir, you are wrong. amps are amps no matter the voltage. come on over and i will show you. my bogart will not lie

I need to chime in here, too..

It's already been said before, but Ohm's law and Watt's law (as in "laws of unalterable physics") indicates that P = VI. This has been explained previously.

Perhaps what you're trying to say is that a wire of a given size can only carry so many amps while maintaining efficiency and that it doesn't matter the voltage. That is mostly true. A 12 gauge wire can happily carry 20 amps over 5 feet with minimal voltage drop. It can carry 20 amps @ 120 volts and it can carry 20 amps @ 12 volts.

The difference is the actual usable power (watts). 20 amps @ 120 volts is 2400 watts. 20 amps @ 12 volts is 240 watts.

2400 watts can run an electric space heater. 240 watts can run a couple light bulbs. Very different.


A 1500 watt space heater running at 120 volts would require 12.5 amps.
TO DO THE SAME AMOUNT OF WORK a 1500 watt space heater running at 12 volts would require 125 amps.


Superdave, as mentioned before, I think you're checking the amperage draw at the wrong location. If you're running a 120 volts AC appliance and measuring amps at the plug then you will see no difference between running off of shore power or an inverter. Measure directly at the batteries and you will easily see many times the amp draw.

Try this: run a 120 volt AC appliance off of your inverter. Measure the amps at the plug of that device (using a kill-a-watt meter or whatever). Now measure the draw at the battery (using a clamp on ammeter). You WILL see a huge difference.

[jazty]

Now, to get back to the question at hand.

If I were looking for a quick way to power a car stereo system without wiring it all up to the bus I would do the following.

- Grab an RV 12v converter/charger such as this 55 amp model: https://www.amazon.com/s/ref=nb_sb_n...s=rv converter
- Get a stereo system that will pull 10 fewer amps than the charger can provide. Hook everything up.
- Get batteries. Hook stereo and charger up to batteries. Plug the charger in when you can.

[PNW_Steve]

Originally Posted by jazty

Now, to get back to the question at hand.

If I were looking for a quick way to power a car stereo system without wiring it all up to the bus I would do the following.

- Grab an RV 12v converter/charger such as this 55 amp model: https://www.amazon.com/s/ref=nb_sb_n...s=rv converter
- Get a stereo system that will pull 10 fewer amps than the charger can provide. Hook everything up.
- Get batteries. Hook stereo and charger up to batteries. Plug the charger in when you can.

Good advice.

If you can find any details regarding actual current draw of the stereo gear that would be helpful in making sure that you size the batteries and power wires properly.

Twigg's suggestion of using the kill-a-watt would be perfect if you were running home (120v) stereo gear but, unfortunately, will not work with a 12 volt battery powered stereo.

[jazty]

Yeah, I would get all the stereo gear together and test it out at it's max bearable volume with a clamp-on ammeter attached to the battery leads.

[warewolff]

Glad to see you're still here jazty! An ammeter is the right idea ... and please OP, for the love of God, do not take any advice from superdave.

Originally Posted by superdave

for maximum battery life you do not want to drop your soc past 80 percent. anything past 60 will damage the batteries.

Wrong again. If OP purchases deep cycle batteries, which I specifically recommended earlier, they can be discharged down to 50% without harm. In fact, my heavy duty Crown CR430's can even be discharged past that every now and then. You don't want to drop car batteries past 80%, but deep cycle batteries are SPECIFICALLY designed for deep discharge cycles. OP, it would be in your interest to put superdave on ignore at this point as you'll end up with a dysfunctional or broken system if you heed his advice. This thread has devolved into a pissing match though, and I'm not sure OP is even still reading the replies.

[PNW_Steve]

Originally Posted by warewolff

an ammeter is the right idea ... and please, for the love of God, do not take any advice from superdave. He has absolutely no idea what he's talking about.



Wrong again. If OP purchases deep cycle batteries, which I specifically recommended earlier, they can be discharged down to 50% without harm. In fact, my heavy duty Crown CR430's can even be discharged past that every now and then. You don't want to drop car batteries past 80%, but deep cycle batteries are SPECIFICALLY designed for deep discharge cycles. OP, it would be in your interest to put superdave on ignore at this point as you'll end up with a dysfunctional or broken system if you heed this garbage advice.

More good advice.

[Twigg]

Yeah, we always base our power reserve calcs. on a 50% state-of-charge for deep-cycle batteries. There wouldn't be much point in calling them "deep-cycle" otherwise.

There is a technical argument that 80% SOC would produce a longer life, but as Trojan expects their batteries to last 8 to 10 years on the 50% figure, I'd be happy with that.

Now if you can afford Lithium batteries ... they can go much lower.

Just a note ... there is also a restriction on how fast the batteries can provide that power, but for the average RV that shouldn't be an issue. It's worth thinking about though if you want to run things like induction hobs, etc, from an inverter.

[BigSteveATX]

Originally Posted by warewolff

Glad to see you're still here jazty! An ammeter is the right idea ... and please OP, for the love of God, do not take any advice from superdave.



Wrong again. If OP purchases deep cycle batteries, which I specifically recommended earlier, they can be discharged down to 50% without harm. In fact, my heavy duty Crown CR430's can even be discharged past that every now and then. You don't want to drop car batteries past 80%, but deep cycle batteries are SPECIFICALLY designed for deep discharge cycles. OP, it would be in your interest to put superdave on ignore at this point as you'll end up with a dysfunctional or broken system if you heed his advice. This thread has devolved into a pissing match though, and I'm not sure OP is even still reading the replies.

I'm still reading. I definitely appreciate everyones willingness to help, even if it's not 100% correct sometimes.

Here's how I'm planning it now:
Onan 6500 watt (54 amp) generator powering a 20 amp circuit for the outlets that run to the rear of the bus. > PowerMax 110 -> 12v (75 amp) Inverter plugged in >0 gauge wire to 2 Battery Bank of 1250W 12V High Performance AGM Power Cell Battery > 6 gauge stranded wire to distribution block > 8 gauge to each Class D amp

When the generator is on the inverter should supply the system with enough juice to power it and charge the batteries. When the generator is off, I should have enough power to run the system for 1-2 hours off the battery bank.

Being in Texas, the generator will most likely be on most of the time since it's powering the 2 rooftop AC's, but on nice days like today, when it's 73 degrees out, I want to make sure the stereo has plenty of opportunity to run without the generator on. Also, there will be no shore power connection...I don't plan to sit long enough for that to be necessary.

[warewolff]

Originally Posted by BigSteveATX

Also, there will be no shore power connection...I don't plan to sit long enough for that to be necessary.

Shore power comes standard on many of the inverter/chargers and is a nice option to charge your bus up fast when it's sitting without cranking the generator. Sounds like you've got your bases covered though. Good luck, and post pictures when it's completed!