Determing Amp/Hour Consumption

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Well-known member
Sep 25, 2010
Washington State
I've spent the morning determining my average daily amp/hour useage and thought I was finished until I read some new information. The article stated that the amperage draw on an inverter at 120VAC requires an input amperage at 12VDC of ten times the output amperage.

For instance, my television uses approximately 0.4 amps at 120VAC, so 5 hour of use would add up to 2.0 amp/hours. But would the draw on my battery bank during those 5 hours of use actually be 20 amp/hours?


Yep, actually a bit more due to inefficiencies in the inverter.  Work in watts and it's easier.  Your TV uses 48W, so at 12VDC would use 4A, or 20AH for 5 hours.
I did a little more checking Ned. The specs plate on the back of the tv reads 0.6A @ 120 vac.  I disconnected the positive feed to the inverter and attached my multimeter (on the 10A current setting) and it reads ~2.1 amps when the television is on. When it is off, the meter reads 0.36 amps.

Strange thing is that my little Shumaker inverter reads 38 watts when the tv is off, and 42 when it's on. There should be a much greater difference between the on/off readings, right? I think it's not displaying the true wattage.

Anyway, at 2.1 amps, there would be 10.5 amp hours (measured pre-inverter) of battery drain in 5 hours. Correct?
Plug the TV into a Kill-a-Watt and see exactly how much power it uses when on and when off.  The plate may be wrong, or more likely, a maximum draw that doesn't represent the long term.  Also, the TV will still use power when off, unless it's unplugged.
If I had a Kill a watt I would, but I don't think I'll get one. I think the multimeter reading gives me what I need. I have no problem converting amps to watts, and vice versa.  ;)

While I'm at it, is it advisable to increase my total daily Ah use estimate by say 5% or 10% in consideration of wire resistances throughout my TT?

After learning how much using the inverter drains the batteries, I'm going to try to limit its use. I measured 8.5 DC amps useage (highest reading) when charging my Mac, and I use it a lot, charging it 4 or 5 hours a day.

Here's where I stand so far:
I have a pair of new Trojan T-105s, so I have 225 Ah's of capacity, fully charged.
My estimated daily useage is 90.35 Ah. Add on an extra 5% for wire resistance, and another 5% for unaccounted phantom loads and whatever else, brings the total up to right next to 100 Ah daily use.

I would guess that would be a maximum, especially since I haven't accounted for a reduction to the total from the time I run my generator (which I think I should do). So I'm thinking that it would be best to do a couple hour recharge in the morning and in the evening, to prevent my battery charge from ever dropping too low.

Trojan recommends to never let the batteries drop below 80% charge. Probably not realistic, but I guess it can drastically increase battery life.

Thanks for you help, Ned!
Don't bother trying to factor in wire resistance, for most DC loads the current draw will decrease with higher resistance (I=E/R).  Your best solution is use a monitor panel for your inverter that tracks AH used as well as voltage and current.  Tell us what model inverter you have and someone can recommend a monitor, if there's one available.
SmokerBill said:
I would guess that would be a maximum, especially since I haven't accounted for a reduction to the total from the time I run my generator (which I think I should do). So I'm thinking that it would be best to do a couple hour recharge in the morning and in the evening, to prevent my battery charge from ever dropping too low.

Thanks for you help, Ned!
That's what we do when we are boondocking. We run the gen in the morning while we are fixing breakfast and let it run for a couple of hours. We do the same in the evenings while fixing dinner. This way we are using the gen during our peak usage times so the batteries aren't drawn down and we are charging the batteries for what we used during the night. Has worked for extended times and the batteries have never run down even when using the furnace at night.
Hey Ned,
My inverter is a small 410 watt Shumacher, model XI41DU. According to it's digital readout, it's never been used at over 150 watts of output. I've used the small inverter for a few things, as I said. Phone and Macbook charging, and some television in the evenings.

Just knowing a rough estimate now of how many DC amps it requires to do those chores makes a world of difference to me. It would be cool to track the AH used, but since I can pretty much get by with using it only for the television for a few hours at night, I'm good with just some rough numbers.

I've never really ran by batteries down much at all using it for tv and as a phone/laptop charger power source. By cutting out most of the charging use, I pretty much know I'm using somewhere between 10 or 20 amp hours a day, if that.

Warsw, I'll just do what you're doing for now.

Thanks again!

What you should factor in is that the inverter is at best about 90% efficient, so the DC power consumption will be 10% or more higher than what you calculate for the 120vac load. If the inverter is running at less than peak load, it is probably even less efficient.

Most rating plate loads are peak and not necessarily what the sustained load will be. As you have seen, sometimes not close at all.

Also, remember that your batteries cannot deliver their full rated amp-hours except under very controlled conditions.  At loads above a few amps the total available AH drops quickly, but at loads of 1-2 amps most batteries will yield more than the rating. And in practice you cannot use more than 50-60% of the battery's total AH without shortening its useful life.

If all this makes you think that calculating your battery AH requirement is a crap shoot, you would be right. However, in real life you will quickly develop a feel for how long it lasts.
Lots of good info. Thanks! Gary, I checked the manual that came with my inverter and it's rated at 85% efficiency. Yeah, I think crap shoot is right, or more precisely and educated crap shoot. I've been focused on my electrical system for a while now, what with planning the install of the new battery pair and trying to upgrade the electrical in small ways, one step at a time.

Originally the TT had one group 24 marine battery. I added another soon after I bought the trailer, and that was about two years ago. All was going pretty well until lately. The group 24's just didn't seem to hold as much of a charge as they did in the past, so that's when I decided to upgrade to the T-105's. (After I bought them, I learned about "equalizing" batteries, and figured that could have been what the old batteries needed to increase their capacity. But too late for that; the cores were turned in)

The original positive wire from the battery bank to the converter (25 feet long) was skinny 10 gauge wire, so I upgraded that to #2 cable. I beefed up the size of the shorter grounding runs to the trailer frame on each end and made sure the connections were corrosion free and tight. So my batteries and main wiring are up to snuff now.

Next step is power conservation. I will most likely get a few LED lamps to replace the ones in the TT I use most often. I don't really use that much light at night, usually leaving just the lamp over the range on full time. With an LED there, in the bath, over the dinette and over the bed I be able to hang onto quite a few AH's.

I'm equally, if not more concerned about the health and care of my batteries. I bought a smart 40A remote charger that can give the batteries a good, full charge much quicker than the built-in converter charger can. I try to give the batteries a complete charge a couple times a week. And I try not to let them run down too much if I can help it.

And that brings up another perplexing issue. I've learned that the electrolyte temperature determines the voltage of a full charge. At 70 (or 80 degees, depending on the source), 12.7+ is a full charge. But for every 10 degrees drop, the voltage at full charge is 0.1 volt lower. So for instance, a 50 degree battery would be fully charged at 12.4 or 12.5 volts.  So in addition to paying attention to the loads on the batteries, and to not discharging them too much, you have to consider the temperature of the batteries. There's a lot to think about when it comes to RV electrical systems.

Maybe I'm thinking too much about this. But hey, I guess I got the time, and it's interesting.

Have a good one,

I remember seeing a 3 stage charger that came with a battery temperature module - you glued it to the side of the battery case or something - and it automatically compensated for battery temperature while charging.

But for most cases the simple 3 stage charging setup in the Progressive Dynamics line with their Charge Wizard works just fine.

FWIW, I've found water usage varies in direct proportion to the amount of power being pushed in and out of the batteries.  If I'm boondocking and regularly cycling the batteries down to 50% or 60% of their capacity, I'll have to add water every couple of weeks.

But when I'm connected to utilities, the batteries are just sitting on float and I can go several months without having to add water.

This makes sense if you remember that you're actually running an electrolyzer (electrical current passing between two plates suspended in a water solution) splitting H20 into it's component atoms whenever you run current in or out of the battery.  The quantity of the conversion is in direct proportion to the amount of current passing through the water .
That could be the newer Xantrex chargers, Lou.

My Xantrex is an older model, but it was explained to me that it does have a temperature sensor built into the terminal clamps, so I think it does compensate for ambient temperature. What I really like about the Xantrex is that it can be manually set to its equalization mode if there's too great a variation of SG's between the cells, or if the electrolyte has stratified (low acid concentration on top, high concentration on the bottom). Equalization causes the electrolyte to outgas, and the bubbles effectively mix the electrolyte to a uniform state.

The Progressive Dynamics converter (PD4045) in my TT only goes into its equalization mode when it's been operating in storage mode (13.2 volts) for 21 hours. It does 15 minutes of equalize mode after each 21 hours of storage mode. So while boondocking like I'm doing, it will never equalize the batteries, even if they need it.
Check and see if your TV has the 120v cord running straight into it or if you use a power brick. If it uses a power brick, check the out out voltage, it may be 12v. If it is 12v you can replace the cord with a 12v cord with the proper end and run it straight off the battery, losing the 15% loss at the inverter and more when the brick drops the voltage back down to 12v.
I use a netbook when traveling and found a car charger for it as well as a portable DVD player and our TV.
Jim, my television is exclusively 120 vac- no converter on the cord. And Apple still offers no charger for their Macbooks that plugs into a 12 vdc plug. There are aftermarket DC chargers available, and even ways to hack your own, but I believe the factory Macbook chargers are smart, and I wouldn't feel comfortable bypassing that intelligence. :)

My Xantrex RV2500  inverter/charger has a separate temperature probe, which I stick to the side of a battery (and in between two batteries).  I think most larger Xantrex and Magnum inverter/chargers have this function.

The useful life of a battery, i.e. how long is can take a more-or-less full charge, is one of the major quality measures of a deep cycle battery. Few brands of size 24 or 27 batteries were designed strictly for deep cycle use, so they tend to have fairly short working lives.  They were all adapted from automotive starting batteries, which have different characteristics.  A true deep cycle from Trojan or most any brand of Golf cart battery will be far superior in longevity.
Yep Gary, that's why I bought the Trojans!  An interesting thing about the new batteries that I didn't know before reading the Trojan literature- they don't attain full capacity until after the first 50- 75 charge cycles. They actually require a break-in period to reach peak performance.

Hi Ho:  What you really want to know is the charge condition of your batteries.  So what you want to know is how many amp. hrs. go into the batteries and how many go out.    That way you know if, for example, you need to run the generator and how long.    However, trying to calculate battery draw and guessing how much charging has been done will drive you crazy.  A company called Bogart makes a device called the Trimetric 2025 that measures current (both into and out) of the battery and does the arithemetic for you.  It costs about $200 with the current shunt.
There are several similar items, Trace has some, Xantrex also and I have a TM400 that does the same or similar job.

To me, something like this is needed for proper battery management.
SmokerBill said:
An interesting thing about the new batteries that I didn't know before reading the Trojan literature- they don't attain full capacity until after the first 50- 75 charge cycles. They actually require a break-in period to reach peak performance.  Bill

This is probably true, to a limited extent, with any true "deep cycle" battery, but I strongly suspect it's more advertising hype than anything.  It takes a few hard charging episodes to reach a truly full charge on any battery.

One other monitor brand to consider is Victron.  This brand is used extensively by cruising sailors who consider it to be more robust and reliable in adverse (damp and salt) conditions.  It is also very easy to install.  I am about to install a BMV-600S in my fiver.  It monitors a single house bank.  They make another model which monitors a house bank and voltage of a start battery bank.  There are online US distributors who I will refrain from naming.

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