Another battery Question

[hitthebricks]

I need to replace batterys this year and would like to go to group 29's but can only replace one with a 29 because of door size. So I would have 0ne 27 and one 29, would that create a problem?

Thanks

HTB

[delcecchi]

If it is for a 24v system it won't do any good. The batteries are in series and when one runs out the voltage drops too much. You could even wreck the smaller by discharging below zero and reversing polarity.

[hitthebricks]

Thanks for the input Del

[BobT]

If it is for a 24v system it won't do any good. The batteries are in series and when one runs out the voltage drops too much. You could even wreck the smaller by discharging below zero and reversing polarity.

I'm not sure I follow this based on my experience with power supplies. In my experience what you say would make more sense in regard to parallel wired power supplies but not series wired. Do you have some electrical theory to support this Del? I'm interested. Thanks.

[fishwater]

Del is right. Series is a more detrimental setup for mismatched batteries. The weak battery is a bottleneck for the current able to be sourced by the circuit, the low potential battery will actually appear as an additional load to the higher potential battery.

If you're talking mismatching a used set of batteries on-hand, folks have been doing that since RVing became a mainstream activity and I wouldn't get too worked up about it. If you keep everything charged, things will typically perform OK within a reasonable discharge percentage of the setup. But for a new purchase, you'll get the best life and performance out of a matched set.

[BobT]

Del is right. Series is a more detrimental setup for mismatched batteries. The weak battery is a bottleneck for the current able to be sourced by the circuit, the low potential battery will actually appear as an additional load to the higher potential battery.

If you're talking mismatching a used set of batteries on-hand, folks have been doing that since RVing became a mainstream activity and I wouldn't get too worked up about it. If you keep everything charged, things will typically perform OK within a reasonable discharge percentage of the setup. But for a new purchase, you'll get the best life and performance out of a matched set.

From my electrical design experience, I'm not quite agreeing with what you're saying. Two power supplies in series will add the voltages together resulting in the combined voltage output. In this case 24vdc. If one battery goes dead there will be one of two results.

1. If the dead battery is open or has an extra high resistance, the open battery (worse-case) would essentially be a break in the circuit like opening a switch and the output voltage from the two batteries will be zero.

2. If the dead battery is shorted or at very low resistance, the shorted battery (worse-case) would essentially be akin to a wire and so the output voltage from the two batteries will be 12vdc.

What you seem to be describing is more in line with parallel wired batteries. In my diesel truck I had two batteries wired in parallel. This was needed because one battery alone wasn't capable of delivering adequate current to the motor starter, especially when the engine was cold. Wiring two power supplies in parallel results in doubling the current capacity while leaving the voltage the same. If one of these batteries goes bad, there are one of two results.

1. If the dead battery is open or high resistance, its as if it was removed from the circuit (worse-case) and so the capacity from the power supply is limited to the remaining battery.

2. If the dead battery is shorted we have a much bigger problem. A shorted battery (worse-case) is akin to a wire and becomes a heavy load on the remaining battery.

Here, I'll illustrate what I'm trying to show.

[delcecchi]

I'm not sure I follow this based on my experience with power supplies. In my experience what you say would make more sense in regard to parallel wired power supplies but not series wired. Do you have some electrical theory to support this Del? I'm interested. Thanks.

Yep, all kinds of theory. In series connection, the batteries are wired

+-+- to produce 24 volts across the pair. All current must of necessity flow through both batteries. No place else for it to go. So if one battery has say 80 amp hours and the other one has 150 amp hours, after you use up 80 amp hours the little one is totally flat and the voltage drops to 12. You could continue to draw current but you would be charging the small one backward, discharging below zero. I am told that is a very bad idea.

[BobT]

Yep, all kinds of theory. In series connection, the batteries are wired

+-+- to produce 24 volts across the pair. All current must of necessity flow through both batteries. No place else for it to go. So if one battery has say 80 amp hours and the other one has 150 amp hours, after you use up 80 amp hours the little one is totally flat and the voltage drops to 12. You could continue to draw current but you would be charging the small one backward, discharging below zero. I am told that is a very bad idea.

I don't know. In the 25+ years that I have been designing electrical circuits I have never heard of this. I honestly don't see how two batteries connected in series can do what you describe.

[BobT]

Del,

Thank you for correcting my understanding. I decided to do some additional research myself and found numerous resources to back up what you are saying, although with one small difference. Only one source suggested the reverse voltage charge would occur but all of them did indicate that the lower AH battery will be depleted before the higher rated one and could be excessively discharged. Depleting wet cell batteries too far is never a good thing to do. Most manufacturers recommend avoiding depletion deeper than about 50% give or take.

In addition, when recharging them together the lower AH battery will reach full charge before the larger one and so the charger will be fooled into thinking they are both charged.

NOTE: we typically don't use 24v chargers to recharge our series wired batteries but instead use two 12v chargers in parallel or charge one battery at a time using a single 12v charger.

[randy t]

Im not an engineer,I haven't even played one on tv. With over 35 years as a mechanic in the construction, trucking industry and the same with boats; if its a 2 battery system,replace both, if its a 3 battery system replace all 3. If one went bad the other will follow. You can get by with short cuts but in the long run it will cost you. What's a day on the water worth?

[randy t]

I guess what I was trying to say is that all battery's in system should be equal, age and size.

[BobT]

The system is only as durable as it's weakest link and I wasn't disputing that. I just wasn't aware that connecting batteries in series in that way would be such a bad thing. I now stand corrected.

[randy t]

It will work, its just not optimal. I consider trolling motor batteries extreme usage.

[jmd1]

I worked on fire trucks for many years. We load tested each battery individually at PM. If one battery was lower than the others, they all came out. That is 3 batteries per side 6 total. Delco 50 and 51 series top screw post. We had a cut down bench with a 3 battery set up. We would put 2 good batteries and one bad in the setup. Then do training on how to identify the bad one.

The other thing is that if you have 2 or 3 batteries, one being a bad battery, if you just swap out the bad one, now the others have been starved for full charge and will soon start failing. Before you know it you are in a cycle of changing out 1 battery each time where if you swap all out at one time, you spend much less over time.

[delcecchi]

Del,

Thank you for correcting my understanding. I decided to do some additional research myself and found numerous resources to back up what you are saying, although with one small difference. Only one source suggested the reverse voltage charge would occur but all of them did indicate that the lower AH battery will be depleted before the higher rated one and could be excessively discharged. Depleting wet cell batteries too far is never a good thing to do. Most manufacturers recommend avoiding depletion deeper than about 50% give or take.

In addition, when recharging them together the lower AH battery will reach full charge before the larger one and so the charger will be fooled into thinking they are both charged.

NOTE: we typically don't use 24v chargers to recharge our series wired batteries but instead use two 12v chargers in parallel or charge one battery at a time using a single 12v charger.

To see how the reverse charging takes place, replace the trolling motor load in the diagram with a resistor or a wire. Now the two batteries are connected like this:

+-

-+

The plus of one is connected to minus of the other and vice versa. (through the load, of course). In the long run, one battery wins and the voltage across the other battery gets reversed. If they are perfectly matched they both go to zero at the same time.

I charge my 24 volt system with a two bank onboard charger, so it is like having two 12 volt chargers.

And I never played an engineer on TV, but I did play one at work for a number of years...

[BobT]

Yes. I felt a need to dig further and you are absolutely correct.

My brain must not have been engaged at all yesterday. As you suggested above, I think I was blinded by the load. For some reason, until I took the load out of the circuit I didn't see it.

[jkrash]

Lets say your running two(24v)or three batteries(36V),is it neccessary to brake in new batteries? If so what's the proper way to do this.

[BobT]

I've never heard of breaking in a battery. About the only thing I do when I buy a new one is put a charger on it right away to be sure it is fully charged. It's not good for a battery to sit partially charged for any length of time.