# RV battery protection



## SolarGary (Sep 8, 2005)

Hi,
Working out the electrical system for our new fuel efficient RV.

Based on the RV loads and the desire to be able to be independent from hookups for a couple days, have decide on two 6 volt 200 amp-hr golf cart batteries in series for the house battery (say two Trojan T105's, or a couple of the Costco 6 volt golf cart batteries).

I want to put a VSR (Voltage Sensing Relay) similar to this one: http://www.delcity.net/store/Voltag...B9tiNNDe2K8JaUcaMrIES9MXdnCkkUoLinBoC053w_wcB
In the line from van starter battery to the house batteries.
The idea being that it allows the house battery to be charged by the van but prevents the vans starting battery from being drained by RV loads when the ignition is off. That is, it connects the van and house battery with van running and separates them when van not running.

The alternator on the van (a RAM ProMaster cargo van) is rated at 180 amps, and the VSR is rated at 125 amps. Also plan to have a circuit breaker in the same line from the starting battery to the house battery -- maybe rated at 100 amps?

There has been some advice that the VSR should have a rating higher than the alternator rating. But, I talked to Trojan and they said that the recommended charging current is 30 amps for the T105, and more than 60 amps will cause serious gassing and overheating. So, I am making the assumption that the alternator is set up so that it will have a low enough voltage that it won't be charging the batteries so hard as to overheat and damage them -- thus I don't need to try to find a VSR that is rated for the full alternator current? It seems like the 125 amp rating on the VSR provides lots of margin over a charge rate of even 60 amps, which would damage the batteries.
Does this make sense?

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More on the proposed van electrical system here -- comments appreciated if you have the time to look at it:
http://www.builditsolar.com/Projects/Vehicles/PMRV/Electrical.htm

From the time I wrote this, I'm leaning toward changing to the MidNight KID charge controller instead of the Blue Sky and toward using the VSR described above rather than the battery isolater.

Gary


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## Jim-mi (May 15, 2002)

Consider just using a battery isolator. 
That way the house battery is always seperate from the vehicle battery.
In your neck of the woods should you do cold weather "traveling" you are always going to want to keep the vehicle battery topped off.

I would also question putting the truck battery in parallel with the T105's. They are two different animals.
If you find that "perfect" spot and stay awhile it makes little difference if the 'house' battery goes way down . . . . . you still must be able start the truck.

Consider carrying a Yamaha 1000i generator and a Iota battery charger.......


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## SolarGary (Sep 8, 2005)

Hi Jim,

As far as I can tell, the VSR and battery isolator work in pretty much the same way. They are both a relay in the line that connects the van starting battery to the house battery. The battery isolator has a third wire from the ignition switch that closes the relay when the ignition is turned on, and the VSR just assumes that when the voltage goes above a certain level that the motor is running and closes the relay.

Have thought about just putting in a high amperage manual switch instead of the VSR, which would give me control over when the two batteries are connected -- downside is that if I forget to flip the switch before a high drain night could end up with a flat starting battery.

Agree that it seems questionable to have the house and van batteries in parallel, but that seems to be what is done on nearly all RV's.

Really don't like generators -- especially while out camping.

I think that in most cases the solar should keep the battery up.

Thanks -- Gary


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## Gray Wolf (Jan 25, 2013)

I'll go WAY out on a limb and say that "all" factory wiring for a tow package includes an isolater. Same for any properly installed aftermarket trailer wiring package. 

Put a meter on the pins until you find voltage on the one that is the charging wire from the engine. Leave lights breaks and turn signals off so you know you actually have the hot charge wire to the house batteries. Ground the other lead on your meter.

Turn off engine. Power will go away due to isolater, or some gadget, stopping the flow of electricity to the trailer wiring when the engine is off. Problem solved if there is no power to no wire when the engine is off - you already have an isolator.


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## Harry Chickpea (Dec 19, 2008)

If you draw down batteries to a level where they won't start a car or truck, you have essentially destroyed them. BTDT.

I thought much the same as you when I outfitted our van for camping a few years back. In retrospect (and with the new technologies available) I would do things differently now. 

First, what I did then:
To keep from frying the alternator, I changed it out to one that I had wound to give more amperage. It worked fine, but there is no free lunch, and the idle speed voltage output was just enough under a standard alternator that I went through van batteries more often and had to skip using the van AC when idling on hot days.

I used an isolator switch and #8 welding cable to feed the rear batteries. Eventually I destroyed the switch (too much current).

The two main power drains were a dorm style refrigerator and a microwave. Both worked, although the microwave only gave about 1/2 power with the stepped AC from the inverter. The refrigerator was much more of an energy hog than I had anticipated.

Even with driving a minimum of five hours each day, in a two week vacation I ruined two batteries from draining them below voltage. The inverter had a 10.5 volt low battery cut-out.

When lead acid cells recharge, they take a high current surface charge fairly easily, but really need a MUCH longer lower voltage and amperage charging to refill properly.

Were I to do it again...
1. I'd skip the microwave and just use the burners that sit on top of a propane bottle - WalMart camping +-$10. Converting electricity to heat is wasteful.
2. I'd skip the refrigerator in favor of an insulated ice chest. Bagged ice is readily available and cheap in comparison. It also doubles as potable water. If you want a freezer, a second cooler vented to the outside can use dry ice. 10 lb blocks of dry ice are commonly used in shipping.
3. I would reduce all electrical draws to an absolute minimum. LED booklights are amazingly efficient. A 9" tablet with a few MP4 files is a substitute for tv and can last for hours on a short charge. 
4. 12 volt fans cool quite well. Old computers are an easy cheap source of quiet ones.
5. With a single lead acid battery, you reduce costs, weight, increase mileage. If it can be slightly downsized to a sealed cell, you don't need to worry about venting.
6. A solar panel charger attached to the roof would work much more efficiently at producing the proper long slow charge.


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## Peasant (May 18, 2013)

30 amps @ 12V is about right for two T-105s in series. That gives you a 13% charge rate. Anything significantly higher than that and you can cook the batteries. 

I'm not familiar with VSRs or charging deep cycle batteries with a vehicle alternator, so can't help you there. There's a thread at the NAWS forum on the subject. Apparently once the voltage gets high enough, current from alternator is reduced to zero. But I'd want to test that with a clamp meter.

Any idea as to the max voltage your alternator puts out?


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## 12vman (Feb 17, 2004)

Quote..
*"Apparently once the voltage gets high enough, current from alternator is reduced to zero."*

Exactly. The internal resistance rises with the charge level. Once they are full charged, they draw very little current. If the voltage level is above the full charge level, (~12.7) they will still conduct (Be Resistive) and this is what makes them boil. They need to boil a little to mix the electrolyte and to assure a full charge..

The biggest current demand will be when you first start a charge. The demand will drop quickly as the voltage increases. The amount of current the battery demands to increase voltage will depend on the level of discharge..

Most vehicle alternators that I have seen float ~14.2 - 14.6 volts. This is a good bulk charge level on a 12 volt solar system but just a touch high for a long term charge. It would cause a deep cycle battery to boil a little more than it should but I don't believe this is high enough to damage it. Just keep an eye on the electrolyte levels in the cells.. (And don't fill them up to the rings under the cap or you'll have a mess! Just keep the plates completely covered.)

I carried 2 deep cycle batteries around in my work van and charged them all day from the vehicle alternator while I drove around doing my job. I removed them and took them inside at night when I got home and that's how I started. (Before Solar) The first two lasted 3 yrs. with no issues. I think I only needed to add distilled water twice during that time.. (It's been a while.. LOL)

I'm with Jim-mi. Just use an isolator. There's going to be a .7 volt drop through the isolator so that's less worry about over voltage issues..


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## Peasant (May 18, 2013)

Harry Chickpea said:


> Even with driving a minimum of five hours each day, in a two week vacation I ruined two batteries from draining them below voltage. The inverter had a 10.5 volt low battery cut-out.


I recently learned that the low voltage disconnects on inverters are to protect the inverters, not your batteries (low voltage could lead to high amps, frying inverter). Which makes sense given that 10.5V would likely cause permanent damage to batteries. Certainly should not rely on LVDs on inverters to manage batteries.

Back to the OP, I found a long thread on charging deep cycle batts with an alternator on a boat forum. Lots of mixed opinions. Kind of a rough crowd over there, but I'd lean towards the argument that a large alternator would probably be fine, even if nominal charge rate is higher than you'd like.

I'd personally just measure it directly, and if the amperage is too high, take measures to reduce it. I'd be wary of anything over a 20% charge rate. Trojan recommends 10-13%.


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## 12vman (Feb 17, 2004)

We use some smaller Newmar power supplies with battery back up. Very simple units. Regulated power supply with either a sealed lead acid or a flooded cell battery. When the grid is up, the load is operated from the regulated supply direct and charges the battery. When the grid goes down, a relay directs the load to the battery instead of the A.C. supply..

Between the regulated supply and the battery, there is a diode and a power resistor. The diode avoids power from the battery to back feed into the regulated supply during a grid failure and the resistor buffers the current to the battery when the grid comes back up. (Current is dropped in the resistor instead of the battery) This protects the battery from too much current too fast. The battery gets a slower charge but over time, will fully charge..

The current limiting resistor is usually small in resistance. (5 to 20 ohms) and at 10 to 20 watts. (Wire wound ceramic and they assume a long recovery time) The size of the resistor depends on the battery size and type. Sealed lead acid batteries wants a slow charge (Higher resistance) and flooded lead acid batteries are more forgiving. (Lower resistance) More battery, larger wattage rating on the resistor..

If you have a 180 amp alternator and want to regulate the current to 125 amps to the battery, you need to drop 55 amps. (660 watts @ 12 volts) The actual load from the battery is the unknown and will vary due to the discharge level.. 

At those current/voltage levels, it's not going to take much resistance to drop current. You could possibly have enough resistance in the wiring between the alternator and the battery. (Line Loss) An isolation diode and a resistor and you're good to go..

http://www.ohmite.com/cat/res_ba.pdf


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## Jim-mi (May 15, 2002)

A battery isolator is just two big amp diodes on a heat sink.
There is no "third" wire . . . . No relays.
Coach and house batteries are isolated from each other.
Batteries like Trojan T105's and vehicle "starting" batteries should never ever be hooked up in parallel. . . . .Bad business.
If a RV manufacture does that, does NOT mean that it is good practice.
Deep cycle and "starting" batteries are two different animals.

Gary if you were to witness how very quiet Yamaha and Honda have made the 1000-2000 watt inverter generators you would likely change your mind.
They are super stingy on fuel. and super quiet.
I do agree the cheap 200 dollar ****generators****are an absolute pain.
Having a good generator (Yamaha-Honda) and a Iota type battery charger could save the day when your at a camp site and there is no sun for the PV.

I think what you have been proposing is over complicating an easy problem.
Forgetting to flip a switch is just a problem waiting to bite you in the arce


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## nosqrls (Jun 9, 2012)

You can do this a couple of ways.

Use all diesel/marine batteries and an isolator. (yes you can use a marine battery for starting a gas rv) You can get a agm marine 75 AH battery for about 230$. Will take a 70 amp plus charge.

GM uses a switched isolator on all dual battery 89 -98 c3500 gas trucks. It only closes if the switch is on.

Run a dual alternator setup. Put dual alt.on the engine. You can make brackets yourself (what I did) or have a shop make them for you. And use longer belt(keep a spare with you). Use a self exciting gm/delco alt. (that way if engine runs it charges batt.) You can if you have suspension clearance run second alt. on the rear end off the driveshaft. (racer trick) You will have to make brackets and pulley for this one.
Delco alt. voltage reg. come in different voltages.


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## wy_white_wolf (Oct 14, 2004)

Think outside the box so this may not work.

I see the problem in 2 main parts. Isolating the house battery from the starting battery and limiting the amperage to 30 Amps when charging. I don't see overvoltage as a problem as that is already regulated by the alternator.

The isolation can be easily done with a relay like is used to control the fan in cars. Just wire the activation circuit of it to the switch instead of the temp sensor.

The amperage limit is a little harder and not sure if this would work. Many MPPT solar controllers are output limited. So they could be wired to the starting battey/alternater and only put out 30 amps on the charge side. I'm wondering if there are any PWM controllers that are output limited. Xantrex C30/C35 has been used as a dump controller. I'm wondering what would happen if you installed one using your house battery as a dump load and the starting/alternater as the input. Would that limit the draw to 30 amps or when used a dump controller is it the load that is limiting the power draw? in conjuction with a relay that could solve both problems.

Might need to do a little experimenting. Sounds like right up your alley Gary.

WWW


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## wy_white_wolf (Oct 14, 2004)

Just did some checking and the C35 is not output limited. In such a configuration it would shut down.

WWW


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## wy_white_wolf (Oct 14, 2004)

Another possiblity would be to install an inverter to power a 30A battery charger. That would allow you to have full control of when to charge the house batteries and could double as the charger when shore power is avaible.

WWW


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## wy_white_wolf (Oct 14, 2004)

How about a DC to DC battery charger.

http://www.powerstream.com/pst-pb1108.htm

Only 8 amps but if you needed more then you could parrellel 2 or more of them.

WWW


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## Gray Wolf (Jan 25, 2013)

There are millions of travel trailers behind millions of tow rigs. Mine included. I have never heard of overcharge problems with any factory or proper aftermarket towing package.

If the alternator doesn't fry the starting battery, why should it hurt the house batteries?


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## Jim-mi (May 15, 2002)

Yup . . .remember an EQ charge is just slightly higher voltage charging and batteries like the T105 eat that right up.

Of course the Trojan guy would say no to the full output of that 180 amp alternator.

folks your over complicating an easy issue..........


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## SolarGary (Sep 8, 2005)

Hi guys,

Thanks -- lots to chew on.

The battery isolators apparently come in two flavors. 
The ones with two diodes that have an input from the alternator and then a separate charging line with a diode to the start battery and another to the house battery. They appear to have the 0.6V diode voltage drop in each line. 

The other kind just has a line from the start battery to the house battery that has a relay in it that is activated when the engine is running. Like this one: http://www.sonicelectronix.com/item_28364_The-Install-Bay-IB200.html
There are no diodes or voltage drop for this type.

This is the type I was thinking of using in that I don't like the new isolator being in the circuit at all for the start battery, or the extra voltage drop to the start battery -- although it sounds like the diode voltage drop might be an advantage for charging the house battery (to avoid overcharging).
The VSR was looking at also has a relay.



After reading all, I'm leaning toward:

- Go back to the relay style battery isolator (instead of VSR). 

- Put an about 60 amp circuit breaker in the charging line to the house battery. If it pops this breaker, I know I'm getting too much charge current.
It seems like there is something to the idea that this sort of arrangement has worked OK for a lot of people with trailers and RV's.


- Keep track of the charging voltage to the house battery, and if it appears to be higher than is healthy for the house battery, then insert some resistance in the charging line, and insert a toggle switch switch in the line that controls the battery isolator relay so I can manually disable charging on long drives.

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I will have an about 230 watt PV panel on the roof for charging and at least during the summer this should pretty well take care of the house battery charging. Going to use the Midnight KID MPPT charge controller between PV panel and house battery. Could not do a project without some solar 

Interesting stuff on the DC to DC charger -- will think about that if the charging rate does not look good.

I settled on the flooded batteries instead of AGM. The batteries will be inside in a sealed compartment vented to the outside. The AGM's have a lot of nice features, but are a lot more expensive, and, if you read the fine print, they still have to be vented to the outside. I don't mind checking the batteries once in a while and adding water if needed.
I am really trying to keep the weight down, but it seems like even with a lot of attention to keeping the loads down, I should have the 200 ah of the two golf cart batteries. 
I've been using 80% discharge level for the heavy load day calculations on the idea that Trojan still shows 750 cycles life at 80% discharge, and this seems like a lot for an RV that might only get used 30 days a year.

Thanks -- Gary


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## Gray Wolf (Jan 25, 2013)

Gary, when you say 750 cycles at 80% discharge, are you saying that you will withdraw 20% of the battery charge and leave 80%, and the batteries will last about 750 cycles and then be shot? That sounds kind of low to me.

Our first set of 12 Trojans lasted 9 years and were still doing fairly well. I only replaced them because I finally got paid for a job I did years ago. We got this set of 16 batteries about 3 1/2 years ago. We usually draw the bank down to 70-75 % remaining in the morning and then recharge to 100% the next day.

We seem to be getting a few thousand cycles, not 750.


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## SolarGary (Sep 8, 2005)

Gray Wolf said:


> Gary, when you say 750 cycles at 80% discharge, are you saying that you will withdraw 20% of the battery charge and leave 80%, and the batteries will last about 750 cycles and then be shot? That sounds kind of low to me.
> 
> Our first set of 12 Trojans lasted 9 years and were still doing fairly well. I only replaced them because I finally got paid for a job I did years ago. We got this set of 16 batteries about 3 1/2 years ago. We usually draw the bank down to 70-75 % remaining in the morning and then recharge to 100% the next day.
> 
> We seem to be getting a few thousand cycles, not 750.


Hi GW,
No, the Trojan graph shows 750 cylces with discharges of 80% -- that is, only 20% left.
The same graph shows a life of 3000 cycles if you only discharge to 20% (leaving 80% in battery). I'd do this if it were for my house and being cycled every day, but for the RV it seems like it should be OK to go for deeper discharges?

http://www.trojanbatteryre.com/PDF/datasheets/T105_TrojanRE_Data_Sheets.pdf

Gary


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## Gray Wolf (Jan 25, 2013)

Agreed. The number of cycles a RV battery will go through probably doesn't matter too much as long as the state of charge doesn't get too low and then your furnace kicks in during the night and takes you down to zero or so. Just like a house, you have to take care of them. 

I have 4 different meters on the house system and none on the trailer. Hmmmm. I'll have to put that on my "to do list". Someday.

Every discharge cycle burns up some of the life. Deeper discharge equals more life sucked out of the batteries so I try to work off the top.

Nice link and it agrees with what happens here at home.

The problem I have with RV batteries is that I forget to check the water. Keeping them in black plastic boxes out in the sun cooks out a lot of water. I seem to remember the monthly battery check for the house batteries but do admit to neglecting the trailer ones.


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## wy_white_wolf (Oct 14, 2004)

750 cycles / 30 days per year = 25 years

The batteries will die of old age before the cycling runs out. I came up with about the same conclusion for my cabin. Calculated them down to 40%SOC for a weekend. With using it every other weekend they'll die of old age before they run out of cycles.

WWW


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