Battery Power Management, House Battery, Chassis Battery, etc

Our 'B' has one group 31 size house battery.  (I ASSUME most of you B'ers out there that one battery is the general rule and more would be the exception.)  That has a listed Ah rating of 115 amp-hours.  On our recent 'Great Northern' trip that battery just seemed to not be really putting out the juice.  It surprised me because we has historically always had enough and we had just switched from a group 27 battery (100 amp-hours) to the 31 just two years ago and if anything we should have had even more available power.  This is what I have found out:

1) The $16 digital voltmeter that I got from Radio Shack several years ago to monitor battery voltage ain't what it used to be.  It now has a 0.15 volt error.  This is very significant when the total voltage range one is looking at is about 0.60 volts.  That is 12.6v for full battery charge (~115 Ah available) to 12.0v for 50% battery charge (~55 Ah consumed and time to recharge for maximum battery life).  So with that voltmeter error we thought we had lost a quarter of useable battery power.  Got a new Radio Shack voltmeter - the price is now $19 but I do like the very small size and have it mounted and hard wired on a wall for easy use - If this second sample doesn't last very long I will look for a different brand.

2)  A battery capacity test when we got home showed that the battery really is in excellent shape.  I load tested the battery:  charged battery over night then 100 watts 1 hour, 1000 watts 10 minutes, 100 watts 1 hour.  Let battery rest for half a day and then measured the battery voltage ( at 12.35v ) and the reading was in the ball park for draining about 50% of the battery usable power.

So, some of the problem was the voltmeter I was using.  I thought we had used up half the battery juice when indeed we still had another 0.15v to go.  But my impression was that the battery had even less life then the voltmeter error allowed for.

3)  The modern age...... : :  We were using the laptop computer much more EVERY night then we have ever done in the past .  I really do not know the exact power it was drawing but its power supply says "60 watts".  Most every night we would download from our digital camera 50-75 pictures.  Review them, delete the bad ones and rename the ones we kept.  Joanne would then do our days trip log.  So, we now have the computer on for 3-5 hours a night plus interior lights ( 40w-60w at computer table and 20w reading light by my bed).

So, it isn't the battery but our "wild" power use.  I have reduced bulb wattage in most light fixtures by 50%.  See how that works on next trip.  Also, next trip we will run the laptop off its own battery when we do not have shore power and recharge it during the day when we have the vans engine running and are charging the house battery.

How do you control power consumption in your 'B' when not plugged in to shore power for several days??  Any tips??  This of course is not a problem that the large RV's have with large battery banks and build in generators.

We use some of the van battery power through the cigerette lighter socket and the acc. socket. We also hook up a solar battery charger to the house battery, and finally we start up the engine and charge the battery that way. We usually run it for about 15 minutes.

Steve 

Len,
Have you considered carrying a small second battery & one of those 75-100 watt inverters to power the laptop - maybe even a motorcycle battery? Could charge it while driving during the day, even on sight seeing trips.

We do have a 1000w inverter attached to the house battery.  The van has outlets for a/c and DC.  I am exploring adding a second house battery.  Might be able to make a well for my present battery and the second one.  It would not only provide more power but free up some space in the van.  The present house battery is behind the food pantry and I have to remove a panel to expose it for servicing.  A 2nd battery will also put us back in the range of being able to be without shore power or running the van for 2-3 days at a time with this "modern" life style.  It is very unusual for use to go longer than 3 days w/o using the van (and thereby charging batteries).  Also we do have a spare laptop battery that can carry with us.  We have just gotten into the bad habit leaving the computer plugged in to the inverter whether we are on the road or parked.

We also could get a Honda 2000 generator.  A Honda 1000 would be enough exept for winter and the 2000 could cover all our potential needs.  We could then use the van in the winter w/o being forest to camp at electrified state parks.  But I have always been against generators in forest campgrounds and that would be going over to the dark side.    We would also have issues with secure generator storage.

And of course it is always just fun to play and modify our toys.

That 1000W inverter may be sucking a fair amount of juice too. It has an overhead (0.5-1.0 amps?) even when idle and is far more than you need to run the laptop. That's why I suggested a small 2nd battery and tiny inverter just for the 'puter.

Good point.  Yes, I believe the efficiency of  invertors goes up with load and the laptop really is a low load.  At low loads a good percentage of the power used is going to the invertor and not the  laptop.  Most every place I have 120v a/c I also have a 12v DC outlet.  So it would be very easy to plug in a small invertor for laptop use when not using shore power.

Many laptop computers have 12VDC power adapters available that would let you eliminate the inverter completely.

My Dell Inspiron 1300 does not have that feature, it requires a AC to DC charger.  Can I borrow your laptop for awhile? ;D ;D

Gary,

The no load power consumption of the Tripp Lite PV 1000HF (bought July 2003) is about 18 watts.  I estimate that the efficiency of the inverter is around 70% when a 60w AC load is attached and that the battery must therefore supply about 84 watts.  Rough numbers.  I note the date on mine because they have newer ones out with the same model number but with more features.

18 watts is  1.5 amps @ 12V, so it's no small thing if on for long periods.  I used to have a small 120W inverter to use with the laptop and it was hardwired to an outlet by the computer work area in our previous coach.  It is still in the car for use there or to plug into the 12v outlet in this coach when we boondock.

Well I am slowly working my way through the loads on the van.  I am measuring voltage drop on the battery over 2-3 day periods.  The last thing I have just done is measure battery drain with things "turned off".  The result is a 1.5 watt per hour drain on the house battery.  I looked under the van and the watts were not setting there in a puddle.  The next step in isolating where this 1.5 watts might be going, I disconneted everything that runs on 12 volts except the inverter.  Low and behold the drain went away.  No measureable voltage drop to the second decimal place for the last 3 days.  I was sure it was the inverter. :-\ :-\  The next step is to seperate (pull fuses) the interior lighting from the radio and see where the 1.5 watts per hour is going.  I bet on the radio.

Len,

Interesting discussion.

I have a fiver, 2 six volt AGMs, a charge wizard & a 1000i.  It takes me a couple of hours of generator time in the morning to pay for the evening 'puter & TV time.

>1) The $16 digital voltmeter that I got from Radio Shack several years ago to monitor battery voltage ain't what it used to be.  It now has a 0.15 volt error.

How did you determine that?

Joel

1.5 watts or 1.5 amps

Common "Always on" include the radio (memory) and Propane detectors, smoke/co detectors may or may not use internal batteries. Air conditioner, furnace and such controllers  You need to de-populate the fuse panel to be sure

1.5 watts over a couple days is a really tiny drain and quite normal. We are talking milli-amps of current.  That sort of load comes from electronics in idle (stand-by) mode, the sorts of things that John mentioned.  Your van probably has less of that sort of stuff than a Class A, but just about anything with digital controls, remote controls or membrane-type switches will have a tiny circuit board monitoring for button pushes and/or remembering settings in its tiny memory.

Gary,

That is why I think it is the radio.  The importance depends on how many amp-hrs or watts one can use from the house batteries.  In my case battery.  In approximate numbers I figure my fairly new schedule 31 battery has ABOUT 600 watts available for consumption.  So over a two day ( 48 hour) period when boondocking the 1.5 watt per hour drain would consume about 12% of my usable power.  It would be fine if we had need for the radio.  In reality we mostly use it while going down the road.  So, if the drain is the radio then it is a simple matter to add an on-off switch to the 12 volt radio feed and basically increase my battery size by 12% 

The down side is the radio (aftermarket Jenson AM/FM radio, CD/cassette player) would need to be reprogrammed when we get to home (like we do now when we get the van out of storage).

Yes, my 'B' is very simple no propane, no refrig., battery operated CO/smoke detector.....

I am ESTIMATING power drain by using a 0.60 voltgage drop as representing the battery half power point (about 600 watts consumed).  So, each 0.10v drop represents 60 watts used, each 0.01v drop repesents 6 watts used, etc.

Joel,

I am using my Greenlee 93-606 multi meter as base of reference.  No, I have not compared that to anyone elses voltmeter.  The new Radio Shack meter I installed in the van reads 0.10 volts higher then the one I replaced (still 0.05 volts lower then the Greenlee).  The old Radio Shack multi meter dial was also wearing out.  Touching or lightly pushing on the dial could change the displayed reading.

I am toying with the idea of getting a Honda 1000 watt unit but I do have storage issues with things in the van.  I really like the size and weight of the Honda 1000 BUT on the other hand like the power of the Honda 2000 but not its size or weight.    :-[ :-\ :-[ :-\ :'(    Because they hold gasoline I would want to store them outside of the van but do not any secure storage or "out of sight" locations.

Life is hard.

Len and Jo said:

I am toying with the idea of getting a Honda 1000 watt unit but I do have storage issues with things in the van.  I really like the size and weight of the Honda 1000 BUT on the other hand like the power of the Honda 2000 but not its size or weight.    :-[ :-\ :-[ :-\ :'(    Because they hold gasoline I would want to store them outside of the van but do not any secure storage or "out of sight" locations.

Click to expand...

A couple of thoughts:
1. It seems like a run to town for groceries, sightseeing, going out for dinner would at least partially get your batter(ies) back up.  How long do you sit without moving?

2. When I was planning, it seemed to me that even if you have solar, you're gonna need a generator for the long cloudy day stretch.  If you have a generator, do you need solar?

3. The 1000i is fine for the things you talk about - really it just gets the batteries back up.

4. Mine rides in a tool box in the truck,  When we're in a dry camping mode it sits on the ground with a bicycle chain (really a plastic coated steel wire cable) attaching it to the fiver.  That wouldn't stop a determined thief, but avoiding tempting an honest man.  You might be able to do something similar (the chain I mean).

Joel

I find it difficult to believe that the 'keep alive' current for the radio is anywhere near 1.5A. A hundred milliamperes - maybe. Keep looking.

Len did say 1.5W which would be on the order of ~125mA.  Others inquired about W or A.  That drain is insignificant and is probably on the order of the self discharge.

Yes, I am talking in termss of watts.  Doesn't matter if one is using a 12 volt DC device or 110v AC device.

Now the way I figure it for NEW batteries (and its all downhill from there):

1 Battery.......about 600 watts usable  (B's, travel trailers, pop ups, pickup campers ???)
2 Batteries.....about 1200 watts usable  (large travel trailers, C's, 5th wheels, some A's??)
4 Batteries.....about 2400 watts usable  (bigger A's??)
6 Batteries.....about 3600 watts usable  The number can go on for ever...any A's...bus conversions carry 6 or more???

So the watts available (no. of batteries carried) to consume and of course the length of time you are consuming the power.  My point being that a couple of watts of "wasted" power in an RV that carries a single battery is far more significant then a couple of watts in an RV that has multiple batteries.

The longest we stay in one place w/o moving (ie: starting and running the van) is about 3 days.  Our present limit is the ice in the chest.  It is very easy for us to be 15-20 miles from a town w a store and supplies.  Also these travels are usually along 1.5 lane dusty, washboard, gravel country roads.  So under those conditons the fewer the trips the better.

So that is why I'm chasing after the watts and trying to make sure that I use them for what I really want.