Battery Tender - 2 12V batteries in parallel

[muckinfuss]

Regarding the fluctuating voltage comment: Remove your battery cables from both batteries at the post. You need to isolate where the problem is before you can solve it. Now take your multimeter and note the voltage of each battery. If they are both fine at the first step, then a parasitic issue is somewhere beyond the posts.

If either one or both are low, most likely at least one is shot and is a parasite on the other. The next step to verify this is to hook up a 'standard' battery charger to one, again by itself and see if the voltage is brought up to acceptable levels. Then do the same to the other battery. Any deviation from normal is an indication of a failure of a battery and it needs to be replaced. A 'battery guy' will tell you that the best test is to leave them disconnected from each other for at least 48 hours after bringing up to normal voltages and retest with the multimeter first step. If the voltage drops on one or both after that, they are technically called 'fried'.

[flyfishMT]

Originally Posted by muckinfuss

Regarding the fluctuating voltage comment: Remove your battery cables from both batteries at the post. You need to isolate where the problem is before you can solve it. Now take your multimeter and note the voltage of each battery. If they are both fine at the first step, then a parasitic issue is somewhere beyond the posts.

If either one or both are low, most likely at least one is shot and is a parasite on the other. The next step to verify this is to hook up a 'standard' battery charger to one, again by itself and see if the voltage is brought up to acceptable levels. Then do the same to the other battery. Any deviation from normal is an indication of a failure of a battery and it needs to be replaced. A 'battery guy' will tell you that the best test is to leave them disconnected from each other for at least 48 hours after bringing up to normal voltages and retest with the multimeter first step. If the voltage drops on one or both after that, they are technically called 'fried'.

Thanks great ideas. I have a battery load tester too.

[Screwby]

Originally Posted by CAG

Ok?

As to oil which is off topic but I will go there anyway. I run nothing but Amsoil in all my vehicles, from oil to diff fluid to transmission and have for years.

Lol, I was using it as an example of the rabbit holes you can go down and people’s opinions with any topic.

I run Mobile1 and Lucas in almost everything.

[JimD]

Originally Posted by Screwby

The way it was explained to me was that in charging and discharging from the positive on one end of the bank and the negative on the other side of the bank that the intelligent chargers will perform better and your bank will stay more balanced as you discharge it as well. Especially when sensing voltage and adjusting charge parameters the charger is seeing the bank from opposite ends instead of reading from the first link in the chain then pushing voltage across the whole bank. I think it becomes more important during equalization modes. I believe the theory is in the resistance of the wires and the fact that none of the batteries are going to be exactly identical in the chain you mitigate that by charging and discharging from opposite ends ensuring that you are pulling and replacing evenly across the cells. I’m a certified idiot compared to the people I asked for advise from but it makes sense in my head.
I deleted my long winded post you quoted Jim. I apologize if you took offense, I shouldn’t post while I watch YouTube car crashes. Makes me all confrontational.

No offense taken. I have been known over the years to insert my foot in my mouth when I came across a bit strong. I don't mind being corrected if it is warranted. All is good here.

[JimD]

Some have mentioned putting the charging leads on different batteries because of the resistance of the cables connecting the batteries together. Just for the heck of it I looked up the resistance spec for the 6 gauge cables I use.

A 2 foot 6 gauge cable has approximately .000804 ohms of resistance. You can't even measure that. The formula for voltage drop across a resistance is IxR (current times resistance). Lets say you pumped 100 amps through this short cable, which would never happen. 100 amps across this amount of resistance would drop .08 volt across the cable. 10 amps would drop .008 volts.

If a battery tender putting out a couple of amps can notice this voltage drop I would be amazed. Another thing to think about is the gauge of wire that the battery tender uses. The one I have appears to use about 18 gauge wire probably because there just isn't much current involved. So the resistance of the battery tender is way over the resistance of my heavy cables. Just thinking out loud.

Now here is where I must admit I am wrong, at least if you have a bank of 4 batteries and charge at 50 amps. I looked up an article of this and sure enough it says to separate the leads. That just floored me. They are the experts and I will relent. I still can't fathom any loss with 2 batteries at a lower charge. It just didn't make electronic sense to me.

As for us RVers out there, I think whatever makes you most comfortable should be what you do. As for me, I am comfortable in my set up. Either way the batteries will charge and we will be happy campers. There are other things to worry about.

I will now crawl back under my rock. Apologies to all.

[flyfishMT]

I connected to shore power and checked and it looks like the onboard charger charges much higher than my Battery Tender so I left it on that.



It was still applying a charge when I unplugged to check and after a 1/2 day on shore power and I'm now at 12.8. Does the onboard charger have smarts to not overcharge?

[RetiredOne]

Originally Posted by flyfishMT

Thanks great ideas. I have a battery load tester too.

To get a better idea on the health of your batteries, you might want to consider disconnecting the cables and doing a load test on each one. A voltage test sometimes won't tell the true story, specially right after disconnecting the charger. Putting a load on them, and seeing how they respond, will help.

[JimD]

Originally Posted by flyfishMT

I connected to shore power and checked and it looks like the onboard charger charges much higher than my Battery Tender so I left it on that.



It was still applying a charge when I unplugged to check and after a 1/2 day on shore power and I'm now at 12.8. Does the onboard charger have smarts to not overcharge?

I think most newer converters do a step charge. You would have to see what model you have and find the specs on the net. My charger has 3 steps. I don't think it goes below 13 volts when it is performing any charge. I have included a chart from mine that shows different charge levels.

[Screwby]

As said above the real test will come when you see how long they will hold the charge. A bad battery will still charge in most cases but will lose voltage quickly even under minimal load. I’ve got a crap battery in my cargo trailer/mobile workshop I used to test and build the system until I can afford a nice new one. My solar or onboard smart charger will bring the voltage up to 12.8-13.1v and float it there. As soon as I unhook the charger or solar and use my usb outlets or my led overhead lighting the voltage soon begins to drop. If I kick on my inverter and charge a cordless battery I can bring it down very quickly.

[flyfishMT]

Originally Posted by RetiredOne

To get a better idea on the health of your batteries, you might want to consider disconnecting the cables and doing a load test on each one. A voltage test sometimes won't tell the true story, specially right after disconnecting the charger. Putting a load on them, and seeing how they respond, will help.

I pulled both off, they were both at 12.5. Load tested both, both said OK, and the reading was 500. The scale goes from 0 to 1000, so I'm guessing 50% life left? After testing they were both at 12.4 This gave me the chance to clean all terminals too.

[Jay2504]

I think that JimD has it right about battery connections. With just 2 batteries it probably doesn’t make a whole lot of difference if both output/charge cables are connected to the same battery. When more batteries are added in parallel, like 3 or 4+ things change. His setup with 2 batteries will work (and has) just fine. As to the OP trying to plug in his battery tender into his Solar port,on both the campers that I have owned, the Solar port wiring is reversed compared to my battery tender. In fact I just reversed my wiring on my new Jayco so I can plug in my charger. The easy way to check, at least on my little charger is to plug in the clamps that clamp to the battery into the charger, trace the wire and see which one is the positive lead. Then check the solar plug. Jay

[Cheetah053]

I have two 100amp lithium batteries. I wire positive to positive and negative to negative with 6 gauge battery cables. Works great for last four years.

[Bill Johnson]

Originally Posted by flyfishMT

Guys I have 2 12V batteries wired in parallel. How do I hook up a Battery tender to them? I am seeing conflicting advice in my googling - clip to one negative on one battery, postitive on the other, and then clipping to pos and

negative on one battery.



Thanks

It can be put on any post, negative to negative positive to positive. If you put it on one battery that battery is connected to the other by cables. if you put it on two batteries one on the positive of one battery and the negative on the other it doesn't matter, you just need longer wire for no reason. I would put it on one battery the power will find it's way to the other battery. good luck

[Vfoote3]

following for info👍

[Screwby]

The OPs question has been answered, debated, and answered again with articles written by professionals in the industry with data to back up the advice given. Not sure why posts with “this is what I’ve done and it works just fine for me” keep rolling in. Unless it’s information backed up with proven data from a reliable source the pile on is just kicking a dead horse yes?
We’ve established the difference in charge due to resistance with two batteries is probably negligible. Anything past two batteries begins to show measurable loss and likely issues down the road effecting battery longevity because of unbalanced charging.

[Mahatnmacoat]

Jim is 100% alright. He's 100% right. If I were setting this up on a boat I'd have a circuit breaker or fuse big enough to trip before an over current load. That breaker would be with in a few inches of the first inline positive terminal. And I'd have a make before break battery off / on selector switch. The first boat I ever rewired was in 1980. I did a nice tidy job, everything run though races, well secured, strictly marine grade parts, I was quite pleased. When I threw that make before break switch to ALL, in a couple of seconds all the insulation on all the wires on the boat burned off. A huge grey cloud of noxious smoke roiled out of the boat. From your pictures it looks like you've got it right. The battery switch is a very good idea. A sturdy good quality multi meter is a good investment. The off / on switch and circuit breaker or fuse protection might be over kill on my part but it comes from my mind set. I've been on a boat that caught fire at sea. I don't know that I'm going to retro fit all that on our motor home, if it catches fire along side the interstate we'll just get out. When you find a guy like Jim, generous with his time and knowledge avail yourself to his knowledge.

May I go off topic with a joke? Don't know why it came to mind but, what are the three most useless things on a ship? A painter's ladder, a carpenter's level and a navel officer.