Battery Charging

Originally Posted by ZekenSpider

I know the on board solar systems can help make up the difference but only if you are traveling in the full sun.

My experience with my solar panels is that they provide benefit in most daytime conditions. Cloudy or overcast, I get some charging performance out of them. I don't know if it is the MPPT controller that is making the difference or not but I do get quite a bit of benefit even when it is pretty foggy out. Most of my camping is dry-camping at beaches and on the California Central Coast that frequently means fog until 10A or noon even. We had a recent camping trip to Refugio. 6 days off the grid. There were about 2 days that I would call sunny. The rest were foggy most of the day with only intermittent sun. It was actually freakishly overcast as matter of fact. Nevertheless, we could have stayed indefinitely under these conditions from a battery SOC perspective and we were not conserving electricity at all.

More germaine to this thread is that the solar panels made us power positive while driving with the frig on DC. Prior to the solar panel installation we were probably 2-3A negative while driving.

This is becoming very interesting.

We will be at the Pismo Beach TM rally later this month, seeing as my wife is running it. I tease her about being the Wagon Mistress. That is about a 3 hour tow, very typical of what we do.

This would be a good opportunity to run a test. I do not have a DC ammeter, so at this time all I can do is measure the voltage.

Someone correct me if I am wrong, but this is what I intend to do:

TM is presently in the driveway connected to shore power with the fridge off.

Three days before departure chill the fridge down using 120 volts AC.

Two days before departure, switch the fridge to propane, to remove the load.

Continue running on shore power at home. With the fridge on propane the only draw will be very nominal. This should allow what ever converter I have to charge the batteries as much as it is capable of.

The morning of departure disconnect from shore power and disconnect the batteries with the switch I installed. The batteries will not be in parallel if the switch is off.

At departure time, first thing is to measure the voltage across each of the two batteries with a good volt meter and note the voltage. Because there has been no load and no charge for about 8 hours the batteries should have settled to their "stable" voltage level.

Then get ready to go. Switch the fridge from propane to 12 volts, setting on 5 (but that likely will not matter), close the TM, hook up and go.

First thing after backing in is to unplug from the TV and disconnect the batteries. Do not connect to shore power yet. Put the fridge back on propane.

After about an hour measure the voltage across each battery again. The batteries ought to be stable by then, I hope. Any difference between the voltage measuree at home versus at camp will be an interesting number.

One of these days I will install a Trimetric meter or equivalent. Just one of those things not high on my list. It might be cheaper to get a clamp on DC ammeter first because I could also use it for other things.

See some of you at the rally in a couple of weeks.

[ShrimpBurrito]

It's a good method in theory, but the problem is that I don't think you aren't towing for long enough to make a quantifiable voltage change. Way too many variables.

harveyrv pointed me to this very handy clamp-on DC ammeter at Sears. IMHO, at $50, it's a steal, and it also measures AC current, and other multi-meter functions as well (voltage, resistance, etc.). It even came with a thermocouple and a nice carrying case. VERY handy in working on anything electrical on the TM, and in this case, it will tell in you 2 seconds what your status is while towing and how bad (or good). Someone at the rally may even bring one....I unfortunately will not be there as we are committed elsewhere.

Dave

[Wavery]

Originally Posted by ShrimpBurrito

It's a good method in theory, but the problem is that I don't think you aren't towing for long enough to make a quantifiable voltage change. Way too many variables.

harveyrv pointed me to this very handy clamp-on DC ammeter at Sears. IMHO, at $50, it's a steal, and it also measures AC current, and other multi-meter functions as well (voltage, resistance, etc.). It even came with a thermocouple and a nice carrying case. VERY handy in working on anything electrical on the TM, and in this case, it will tell in you 2 seconds what your status is while towing and how bad (or good). Someone at the rally may even bring one....I unfortunately will not be there as we are committed elsewhere.

Dave

The DC ammeter is super cool if you have solar panels too.

Originally Posted by ShrimpBurrito

It's a good method in theory, but the problem is that I don't think you aren't towing for long enough to make a quantifiable voltage change. Way too many variables.

Dave

So for those of us that only tow for 3 to 4 hours, as long as we don't leave the fridge on 12 volts during a dinner stop, it won't really matter.

[ShrimpBurrito]

Originally Posted by PopBeavers

So for those of us that only tow for 3 to 4 hours, as long as we don't leave the fridge on 12 volts during a dinner stop, it won't really matter.

I'm not sure I would say that. In my case, and in the case of at least a few others that have posted here, there is a 2-3 amp draw on the TM battery while towing with the fridge on DC using the TV OEM wiring. So on a 3-4 hr drive, you lose 6-12 Ah. If you have 2 Trojan T-105s with a useable capacity of ~140Ah (total capacity of 225 Ah), that is less than 10% of your power. Not a huge deal, but still significant. And all that assumes you don't stop for fuel, food, potty stop, or anything else. It also assumes you turn the fridge on only immediately before turning the TV on, and then turn it off immediately after disconnecting from the TV. Most people don't do that.

But if you only have the battery that came with the TM, or something similar, you only have 80-100 Ah total capacity, or ~40-50 Ah of useable power. In this case, you've lost 20-30% of your power, which is quite significant.

What I am saying in that in either case, I'm not sure measuring voltage is going to be a reliable way of determining the state of discharge. I don't think you'd be able to tell anything using a voltmeter in the first instance, and in the second instance, if you can tell anything, you're only going to tell if you are losing power. You won't be able to tell how much.

Dave

[Wavery]

Originally Posted by ShrimpBurrito

What I am saying in that in either case, I'm not sure measuring voltage is going to be a reliable way of determining the state of discharge. I don't think you'd be able to tell anything using a voltmeter in the first instance, and in the second instance, if you can tell anything, you're only going to tell if you are losing power. You won't be able to tell how much.

Dave

You're right because a voltmeter can only measure surface charge. Realistically, one would have to wait about 8-12 hours (of inactivity) for the battery to balance out. That is hardly the case while we are camping.

Voltage between 11.5 and 12.7 mean little while a battery is either been charging or discharging within that last several hours. After 8-12 hours of rest, 11.5 is totally discharged and 12.7 is fully charged. A fully charged battery may read 11.5 if you are running a heavy load (i.e. microwave off the inverter). A dead battery may read 12.7 while hooked up to the TV with the engine running.