A battery question for Duane - bit off topic

[hal]

Sorry for posting a boat question...But this is the place I've seen battery technology explained clearest.

Now. I bought a sailing boat, '81 modell Vindö 32. Swedish make. Fibreglass hull, mahogany cabin and teak deck and cockpit. Volvo Penta MD7 diesel engine (13 HP)
Now to the core, the boat had two 75 AH batteries, one for starting and one for consuming(fridge, lights). When at dock there's an automatic tender connected. only starter battery was charged...
The batteries are old and dead and I want to do this right. What I read is that the common thing to do is to have one start battery and one consuming bank of batterie(s) and leave it all on one tender. This is where I remember Duane talking about not to put two batteries on the same tender. Should I add another tender to it to make it right? What about when motoring? Only charge one of the batteries at the time?

Thanks for you time and patience.
/hal

[Major Softie]

I am not Duane, and I'm certainly not Diane, but here's what I know about this:

1. You can use a single charger on a group of identical batteries just fine. This is done all the time. It is when you try to create a bank of batteries with different characteristics that it doesn't work well.

2. Are you sure you're system is 12v? Many marine systems are 24v, but the size of your boat is kinda on the line as far as I know.

In a system with 2 batteries with one operating the starter for the engine, usually they are separated by a "battery isolator" which allows both to charge together while isolating the starter battery from the cabin load so that there's no chance of losing your engine battery because you ran your lights and heater too much overnight. This is also how RV's are hooked up all the time.

[Duane Ausherman]

It takes a special charger (tender) to charge two batteries at one time. I would imagine that the original system had an isolator, as MS said.

If you buy two identical batteries, they probably will have similar characteristics and should charge nearly equally on an old standard simple charger. As they are used in different capacities, they will age differently and require a different charge rate for each.

The ratings for batteries are only good when brand new. From there on they deteriorate until they fall below serving the intended purpose. What we see is that the battery starts our car just fine until one day it doesn't. It is natural to think that something bad happened overnight. Not usually so, at first the battery was able to do the job with a lot of spare capacity that wasn't needed. You won't know that, you will just know that the car started. One day the capacity has dropped below what is required and it seems to have all at once failed.

We now have some exotic systems for testing capacity while they are in use. We have them in some cell sites and they seem fairly accurate.

The old way to test was to do a load test, which is in itself abuse of the battery. Suppose that the load test wasn't harmful and you could perform it every day, or week. You could then see that it is dying at a fairly linear rate. If you knew what capacity that you needed, then you could accurately predict the week that the capacity would diminish so far that you couldn't start the car.

Look into the isolator issue and maybe you already have one on board. Those tenders that can charge more than one battery at a time have the isolator inside, or similar.

Huge advances have been made in recent years and my knowledge is related to lead acid types (old) and the specific ones that we use in wireless. So, as we all are, I am limited in knowledge. I bet that someone steps in to add more info.

At cell sites our stack of batteries are connected in series. They are all identical when new and they charge fairly evenly when new. As they age, it is at a different rate. Were they in parallel, it would be a disaster and they wouldn't last nearly as long.

[Major Softie]

Ya, my scissorlift, for instance, uses 4 large 6v batteries in series for 24v, but they are just good modern designs of deep-cycle lead-acid batteries. Still, they last us about 10 - 12 years.

[hal]

Thank you both!

Since this was set up porly by some previous owner and bot batteries are shot, I think I'll try to do it in a right way. Thinking about putting one battery in for consuming and one for starting. Put in a extra tender since the one in there is single purpose. Then put in a switch from the generator so I am able to switch between charging startbattery and consuming battery when motoring. The common thing to do is to put a series of batteries in parallel to form a bank. But as you say that will downgrade lifetime for them. So it might be wiser with one 120A than two 75A. And it's a 12 V setup. What you think?

thanks!

[Duane Ausherman]

If the two batteries are identical, then you could also swap them yearly to average out their life. That might take some rigging to make it easy, but that is all the better. Suppose that you are out away from your berth and one battery goes dead, you can swap them to get back on the road/water.

Having the extra tender is also redundancy. If you go long periods of non-use, I would put the tenders on timers too, unless they have some sort of automatic thing already. If the tender (charger) is the right size of older analog variety, they are already adjusted by the nature of the limits of the size. Just keep in mind that the battery capacity decreases in a mostly linear fashion, if properly attended. That means that the ideal charge rate also decreases with decreasing capacity. I use an old fashioned Variac to be able to adjust the charger voltage, therefore the current. Does your tender have an ammeter to show the charge rate? If not, I always add in one.

Just be sure to coat the battery terminals with NO-OX of some sort. I prefer to make my own cables too. I do a better job of it than those commercial ones. I always use shrink tubing on crimps. Humidity is your enemy, keep it away from the copper. Grease is your friend.

We just had this issue come up on the slash 2 forum. It is interesting what some people believe. Science isn't very popular among some folks. When dealing with electricity, leave religion at home.

The batteries with which I deal are two stacks of 12 cells. Each cell weighs some 200 lbs, more or less. The floors are specially made to hold the dead load. The amount of current is hard to fathom. It is at least 10 times what is available from a large DC welder. We use 4/0 cables, and they are about the size of a finger and they still run warm. We are supposed to use special tools that are insulated. The official tools are quite bulky and hard to use, so I make my own and so far no safety inspector has said anything. Oh, yes, I am the safety inspector.

It is fun teaching the young folks about the safety aspects of batteries. In showing them about the vast current, I make a big deal of removing my watch that has a metallic band and putting it in my pocket. Then I gingerly reach into the huge charger and touch one buss bar and identify it. There are two, positive and negative. Then I carefully reach in and touch the other one. All the while I am building up drama with the talk of this being 10 times the capacity of a welder. They are always quite scared at this point. Then I reach in and grab both buss bars and have the full capacity across my body. They always jump away expecting a lightning bolt to hit me and hurt them too. When nothing happens, they are shocked (pun intended) and amazed that I am still alive. I explain, it is only 24 volts DC and my skin resistance is so great that no current flows.

I explain that there is huge danger, but only if one flashes a tool across the two buss bars. It has happened. One of our bosses did it once and never told anybody. I found the evidence a couple of day later and his name was in the log. Molten metal had blasted all over the charger. It was on the far side where it was hard to see. He didn't clean up, so I knew to look at the backside. Pieces of metal 1/4" in diameter had blasted out of the flash. It must have been exciting. He had to have burned his hands/arms, but we never heard about it. The protective plastic cover, the one to prevent this kind of accident, was ruined, so he must have made it disappear, as we never found it. He was well known to be a liar and braggart. He was hated by everybody on that contract job. Later he tried to fire me and in turn got fired. The "client" knew what was going on.

We learned later that he had sexually harassed one wonderful (and beautiful) contract worker. I wish that had gotten back to us earlier. One of the techs was still a navy seal.

The danger is of a burn, not a shock. Sorry for the long ramble.

[Major Softie]

If he is using real "Battery Tenders," that is a brand name for a charger that is about a 1.5A charger that turns itself off when the battery is full, and doesn't turn back on until the battery's voltage drops to a certain level. Thus, it only comes on when needed.

Hal, make sure you get the correct Battery Tender for the batteries you get. I know that especially some gel batteries need a specific Tender. I'm not sure if it is the charge voltage that is different, or the dropped voltage at which it turns itself on, but one of those is more critical on gels (I think it's charge voltage).

[Deleted User 287]

Sorry for the long ramble.

I love your rambling, Duane. I always learn something.

[hal]

me too Ramble on Duane!

I'll check the charger I have. And it seems like miss Fortuna is looking my way. A friend of mine has some contacts with a major battery guy - I can seek professional (battery that is)advice and get his % off.

[BobW]

If stored in a wet slip, the bilge pump may run occasionally. For this reason, the battery will be depleted unless charged. My owners manual says to use the switch and crank and charge one battery at a time when under way (power boat). When moored, I always switch to one battery and then use the other to crank the motor in the morning.
Ideally, a boat should have both a house battery and a cranking battery, the two applications are different. Being able to discharge the house battery to a low level while on the hook and then recharge while underway or at a dock is imperative.
To use a battery tender, why not get two and leave the Perko off. This leaves a working bilge pump but isolates the batteries while in the slip.
West Marine catalog has a good write up on both deep cycle and starting batteries.
Please remember to use an isolator to prevent electrolysis.
Bob