RV Alternator Charging of Solar Batteries - question

[garybeck]

Hello

the Solar Bus has been going strong with our on-board PV system that includes 8 golf cart batteries. until now I have not had a reason or desire to charge with the alternator while I'm driving. Because our project is educational, I want all those electrons in the batteries to be from the solar, and there's always been enough.

lately however there are times when a little alternator charging is desired. So I'm going to set something up.

I *don't" want to use an automatic battery isolator tied into the vehicle's key because with those setups, it's *always* charging from the alternator when the vehicle is driving. I want it to be manual, so I only engage the alternator charging in the rare times when I need it.

I'm thinking all I need to do is run a wire between the positive terminals of the starting battery and the aux (solar) battery bank, with a manual switch (and a fuse wouldn't hurt). I could go from the alternator to the solar battery but I think it will be easier to go from the starting battery to the solar battery. Since both batteries are grounded to the same vehicle chassis, I don't need to run a negative wire. The wire and fuse should be sized according to the maximum amperage of the alternator. I'll use one of those big fat "knife" switches that can handle the current. I could use this not only to charge the solar batteries with the alternator, but also if the starting battery is dead for some reason I could give it a "jump start" by just closing the switch, connecting it to the solar batteries.

I don't have that much experience with RVs and I'm wondering if anyone has set up manual battery isolator like this and if there is anything I should take into consideration, or if there's anything wrong with my idea/plan.

Thanks
Gary

[Mike90250]

whoa ! Installing JUST a switch and fuse will be bad - if either battery is more than a volt or 2 higher or lower, it will dump full amps (30-300) into the other battery, so you need to be sure your wires can handle that.

Also, a vehicle alternator is only set to 13.8V, and deep cycle batteries are charged at about 14.6V, so, the alternator could "bulk" charge the RE batteries to about 80%, but then would fade off.

Maybe a limiter resistor, possibly a headlight bulb, I use a live high beam, from a burnt out low beam sealed bulb. Lots of current, bulb lights up bright, as batteries approach equilibrium, the light fades as amps decrease, then you could "engage the switch". Blue Seas (boat supply houses) has great battery bank switches, and also makes a nifty battery terminal fuse block

http://bluesea.com/category/5/21/pro...e/overview/378

Good Luck

[Sunking]

I *don't" want to use an automatic battery isolator tied into the vehicle's key because with those setups, it's *always* charging from the alternator when the vehicle is driving. I want it to be manual, so I only engage the alternator charging in the rare times when I need it.

That is just plain silly and illogical captain, and a complete waste of energy not taking advantage of your engines power. You want your battery at full charge 100% of the time. Otherwise you will just shorten the battery life and waste your time and money.

[garybeck]

thanks Mike, I will consider what you said... the switch I have is rated for 200 amps I think. but I'm thinking.... how is this any different than just getting a "real" jump start? I mean when you connect jumper cables, there's no fuse and one of the batteries is presumably dead, so are you saying there's up to 300 amps flowing in common jumper cables every time someone gets a jump start?

as far as the voltage goes, yes I realize the alternator is not going to top off the batteries per se, but the voltmeter on my dash typically floats at ~14 so I'm thinking it will help a lot more than what I currently have (nothing). And since this would be the float (not bulk) I think it would be fairly effective.

certainly I'm not the first person with an RV to charge their aux batteries with the altnerator. most people just buy one of those isolators for ~$15 and let it flow when ever the engine is running. I don't think those things are rated for 300 amps?!?

this is how I think most isolators are set up:



I'm basically going to do the same thing but replacing the automatic isolator with a manual switch.

Aren't I essentially just installing jumper cables inside the bus, that I can connect and disconnect with a switch? My cable will be as big (or bigger, I think I have a roll of #4 wire somewhere) as typical jumper cables. I think I should be OK. You don't agree?

[Mike90250]

Jumper cables are made to handle the high amps, and yes, there are often hundreds of amps flowing thru them. If they are too small, they melt! And you need a switch that wont melt either. 200A is sketchy, a low battery will give it grief.

[garybeck]

That is just plain silly and illogical captain, and a complete waste of energy not taking advantage of your engines power. You want your battery at full charge 100% of the time. Otherwise you will just shorten the battery life and waste your time and money.

I disagree. If you understood the nature of this educational project you would know what I'm doing. I frequently use the energy in the batteries to power a stage, and all the people involved want it to be solar energy that is powering the stage, not diesel or even biodiesel / vegetable oil. we have a sign we hang off the stage that says "powered by solar energy" and that would be a lie if I was using the alternator all the time to charge the batteries.

most of the time my batteries are full anyway, without the alternator. It is only the rare occasion that I need any juice from the alternator.

also I think your premise that batteries want to be floated at 100% all the time. Just like being equalized (overcharged) every once in a while, they do like to be cycled down every once in a while too.

I appreciate your input but I don't think you understand this project and what we are doing here. it is not silly or illogical. (and I think you could find a little more respectful way to communicate if you tried). If I used an automatic isolator, it would be like rolling some grid-power charged up batteries to power a stage and telling everyone it's solar energy when it's not. This is an educational project, and it's a stand-alone system. I only want the alternator for rare occasions.


thanks

[Sunking]

The switch I have is rated for 200 amps I think. but I'm thinking.... how is this any different than just getting a "real" jump start?

There is no difference. If the vehicle battery is dead, there will be a huge rush of current from your auxiliary battery to charge it up. If the cable is too small to handle it it will burn up unless you have some sort of over current protection device and regulator. Vice versa is your aux battery is dead, and you connect it, same thing happens.

This is why I am telling you this is a bad idea. Battery voltages have to be equalized before connecting in parallel. Otherwise you will have a huge transfer of current that you cannot control.

I mean when you connect jumper cables, there's no fuse and one of the batteries is presumably dead, so are you saying there's up to 300 amps flowing in common jumper cables every time someone gets a jump start?

Yes and no. There are a lot of factors mainly the jumper cable size, and the connection resistance of gator clips. The high resistance connection resistance of the gator clips limits current flow to a safe level

as far as the voltage goes, yes I realize the alternator is not going to top off the batteries per se, but the voltmeter on my dash typically floats at ~14 so I'm thinking it will help a lot more than what I currently have (nothing). And since this would be the float (not bulk) I think it would be fairly effective.

Well the alternator had better regulate the voltage or it needs replaced.

certainly I'm not the first person with an RV to charge their aux batteries with the alternator. most people just buy one of those isolators for ~$15 and let it flow when ever the engine is running. I don't think those things are rated for 300 amps?!?

No they are not rated for 300 amps. However they have solid state regulators built in that control the current, and prevent the AUX battery from back feeding the engine battery. Some of the better units have an emergency switch to allow the aux battery to charge the vehicle battery.

Here is the likely scenario of what will happen to you some day. The Aux battery will be deeply discharged, you throw the switch to charge it up, you see smoke, the switch contacts will weld shut from excess current and you will not be able to disconnect, and a fire will likely start.

[Sunking]

also I think your premise that batteries want to be floated at 100% all the time. Just like being equalized (overcharged) every once in a while, they do like to be cycled down every once in a while too.

That would be incorrect Sir. Each cycle takes a little thin slice off the battery plates. There is a huge difference between a float charge and equalize charges. I am a profession electrical engineer of 30 years who specializes in Telecom battery plants, power generation, generators, UPS, distribution who slept at a Holiday Inn last night.

[garybeck]

Thanks Dereck.... so it sounds like you are more in favor of the battery isolator. So let me ask you this... Couldn't I hook up the battery isolator and just put a disconnect switch on the side going to the AUX battery, so it doesn't charge unless I want it to? Then it will still have all the protection of the regulator you refer to inside the isolator but I can still only engage it when I want.

[Sunking]

So let me ask you this... Couldn't I hook up the battery isolator and just put a disconnect switch on the side going to the AUX battery, so it doesn't charge unless I want it to?

Should work OK.

[garybeck]

Should work OK.

Thanks Dereck! I'll go that route! This is exactly why I asked... I appreciate your help.

[Mike90250]

Hi, Sunking. I have 4 big surrette batteries with my solar system. I want to put them in the back of my truck permanently, and charge them everyday when I commute to and from work. and plug them into my inverter and run my house for the evening. You say a battery isolator, what am I going to have to do alternator?
Look forward to your response!
Tyler

This has been moved to the other thread
http://www.solarpaneltalk.com/showthread.php?t=2461

[longwolf]

Hey Gary,
A couple of ideas for you.
Harbor Freight is a good place to get your battery disconnect switch. I got one on sale there for 2 or 3 dollars.
A guy on another forum recommends using welder cables.
They're repetitively cheep, tough, weatherproof and made to carry high current.