What are the benefits, if any, of hooking up a third battery to a 24 volt system. Running in parallel to one of the other batteries, does the third battery offer anything to the system as a whole? It’s got to help something, I’d think, but I can’t get past the fact that its only increasing the capacity of one of the 12 volt batteries, so might not necessarily help the 24 volt requirements at all.
It would:
A) not charge correctly.
B) wreck the lone 12v battery if the system was discharged too much.
Don’t do it.
Damn. Okay.
BTW-
This is why electronic devices admonish you not to mix different battery types. If you have a battery with twice the capacity of another in series with it (which is basically what you would have with your two in parallel), once the lone battery is fully discharged, the big one still has plenty of juice, and it will force current through the lone battery in reverse, which means a painful death for it.
Thanks. I’m glad I asked as I had only installed it a few hours previously. So there shouldn’t have been any harm done. I didn’t really even run it. I might grab a fourth battery in the near future and reinstall them as a 4 battery 24 volt system. As I was pulling the battery out, I noticed that it was an ATGM and the other two were lead acid. That’s probably not a good idea to mix either, right? Or is that okay?
Also, I think you helped identify the cause of some of the problems I’ve been having. I have a power converter designed to operate on 12 volt system. That converter is only hooked up to one of the batteries, so I guess it is not draining the batteries evenly? I think that might be the issue.
Thanks for the help!
To get the maximum economic value out of your lead-acid solar-system, you should charge the lead-acid batteries right up. And add water every month or so.
If you put your gell-cell batteries in parallel while you do that, you can’t add water to them every couple of months, so they’ll die before their time…
Conversely, if you use a gentle charge rate and charging voltage on your combined system, you will never get the lead-acid batteries as full as you could. Batteries last better when fully charged, and you can’t get as much use out of them if they aren’t fully charged, and you may let them discharge too low if you’re trying to get more power out of the system. If you let them get too low, they will certainly die before their time is due.
If you connect them all together, and just try to keep the voltage not too low, not too high, and not use them very much, then they should last ok. But eventually one or the other will die, and you won’t notice it, and that will kill all of them. And in the mean time, you wouldn’t get much benefit, because you couldn’t use very much power.
In summary, the really bad thing about mixing gel-cell and lead-acid isn’t that it doesn’t work: it is that you do your money quicker and don’t get good benefit.
It might also wreck the battery you put it parallel to. Between different chemistries and different degrees of charging, batteries will often differ somewhat in their voltage. For a nominally twelve-volt battery, I wouldn’t be surprised to see variations of as much as a couple of volts. When you’ve just got batteries in series, this is no big deal: At worst, something designed for 24 volts that’s only getting 22 will probably just run a little bit slow, or the like. But now, put two batteries with different voltages (say, 10 volts and 12 volts) in parallel to each other: You now have a potential difference of two volts across nothing but the (hopefully very low) internal resistance of the batteries. A decent voltage across a very small resistance means a lot of heat, and worse, a lot of heat in a spot (inside the batteries) where you really don’t want a lot of heat.
I might be interpreting your statement incorrectly but it I don’t see any way that a larger capacity battery in series with a drained battery could cause a reverse flow in the drained battery. The large cap battery would have to be in parallel to do that.
In series the healthy battery would discharge through the drained battery into the load which would now be acted upon by 12 volts, not 24. That may be bad for the load or it may cause the drained 12 volt battery to corrode depending on the type.
“ATGM”? Anti-tank guided missile?
You’re wiring a TOW in parallel with two lead-acid batteries? 
Think about it:
You have (3) 12V 1A•hr batteries, fully charged. Two of them are in parallel, and these two are in series with the last battery.
So, you have essentially a 12v 2A•hr battery in series with a 12v 1A•hr battery. When you have extracted 24 A•hrs from the battery pack, the single 12v battery will be essentially dead, while there will still be 12 A•hrs left in the parallel batteries. So, the current will continue to flow through the dead battery, charging in in reverse (+ to -, instead of + to +).
From:
lol. AGM. You can tell which acronym I use more often.
In your cite the narrator only mentions that the batteries are in “a pack”. The explanation appears to be a discussion on the dynamics of a parallel configuration. A weak cell in a series configuration will have a lowering voltage and a higher impedance as it drains. The result is that the cell’s resistance becomes so high that it acts as an open circuit. The circuit dies even though there is a healthy battery in line behind the open circuit.
Sorry, but that’s not the issue.
In the case we are talking about, all the batteries are initially healthy, and they all have the same capacity. Their internal resistances are the same. The problem is during discharge - what happens to the lone battery when it is completely discharged?
To elaborate:
In the three-battery configuration, you have (theoretically) 36 W•hr of energy available (if each battery was 1 A•hr, and 12v). What happens when you reach 67% depth-of-discharge, 24 A•hrs? Remember that the lone battery only can contribute 12 of that.
I’ll tell you: The two batteries in parallel still have 12 A•hrs of energy left, and the lone battery is completely discharged. The voltage immediately falls from 24v to 12v, and the lone battery gets wrecked as it starts getting reverse-charged.
beowolf - We appear to be explaining the same effect from different reference points.
My initial concern was when you said " it will force current through the lone battery in reverse" which it will never do.
I see that you meant that it will “charge it in reverse” which seems to me to be an odd way of saying that the current is still going in the same direction.
We agree on the result. No improvement occurs and quite possibly damage to the lone battery will occur.
If nothing is wrong with the batteries, they all lose voltage at the same rate. Some where below 22V lets say, your powered toy stops working. No damage to the batteries. None of them have gone to 0 volts
The bigger battery ( the two which are parallel is just a bigger battery, it does not have more voltage ) The voltage drops in all three together. The bigger battery can deliver more amps than the lone battery in series but since the toy will stop working at 22 or 18 volts, there is no more current because 18 volts is not enough to push more electrons through the system.
You are making bad assumptions.
You have no idea what the voltage input range is. For a simple motor, it may draw current until the batteries are dead.
GusNSpot
Your voltage profile statement is correct but you overlooked battery resistance in your comments.
All batteries assumed equal the single battery will have twice the resistance as it discharges than the parallel set. At best it will discharge to the point where it acts as an open circuit. At worse the current flowing through it from the lower resistance source will cause it to heat up and be damaged or destroyed.
Fine, what ever. I am coming from having operated systems just like this and from teaching how twin engine aircraft run 24 volts with 4 batteries that are never truly equal and also the paralleling circuitry to make that all work. As an A&P mechanic, I have much experience with stuff such as this and in the real world you do not get to the condition you describe. The system shuts down before that happens.
If you just take 3 batteries (lead acid auto batteries) and one high current load that had conductors that can handle hundreds of amps and have no circuit protection and wires don’t melt, batteries don’t explode and stuff & such you may come close to what is being said but that scenario is so far out of touch with what actually happens that there is no point in me trying to explain.
So the best plan is to never do it, just call AAA with your always can connect cell phone and be secure in knowing you will be saved in time.
I would love to see an actual usable load in an actual system that does not run on batteries alone and needs a lot of amps to start the system and then have a charging system to keep the battery up and why you can jump it with the arrangement folks are talking about. 4 12 volt batteries is better but if you don’t have 4 , the boss is gonna be pissed if you all go home or sit around waiting for AAA because you know that three can’t work, can’t tip things just enough to get your start.
There is not a real question here IMO. Just folks wanting to play theory games. The funny thing is that they can’t build a system as they have described it and make it do what they say it will.
They really need to go and work at a not so top notch road building crew and learn how to get it done.
Lets try this, it is a cold nasty day on the new road and the on board battery will not deliver enough amps to make the starter turn the engine. In normal conditions 500 amps will turn the big diesel engine but in this weather, the thick oil and whatnot and the less that perfect huge 24 volt on board battery can’t put out the 800 amps needed this day.
Things to think about:
What does the starter motor need, volts or amps?
Which way the current is actually flowing. ( does not make any difference you say, we’ll see.)
A starter motor has such big windings that it is really a direct short before it starts turning.
Back EMF is what reduces the current flow or if you prefer to think of it as resistance, that is cool. The faster the starter spins, the harder it is to push amps through it. Because of Back EMF . (Electric Motive Force)
No way to preheat and only 12 volt pickup trucks around, 2 battery chargers that can put out 50 amps each and you have the extension cord to plug them in with.
Only three guys are willing to put their trucks at risk for this try at a start.
So, you take 2 batteries from the pickups and unhook them and make a 24 volt battery by hooking them in series and add that to the big 24 volt on the equipment. In parallel of course.
The starter turns slowly and won’t spin fast enough to get it to engage. Hummmm we have big jumper cables and there are no real hot spots so…
We add a 50 amp charger to one of the 12 volt batteries and the other to the other battery. This in effect makes two bigger 12 V batteries and due to the way they are hooked up on the equipment you get even more amps delivered and the starter spins even better , it engages but won’t spin the big engine fast enough. Need more amps. Not volts, amps.
Take the third battery and hook it in parallel to the battery that is closest to the negative connection on the starter. It will not up the total voltage but we don’t care because we want amps and we can’t deliver them because the batteries can’t produce them, even with the plug in chargers helping deliver amps.
Remember the starter need amps to make the force field that will make it strong enough to spin the engine fast enough to start. The starte does not care where the help come from, a hand crank might just make the difference but the equipment is not set up for it so it must be done electrically.
( On WWII aircraft with hand crank & battery crank inertial starters that you can wind up if the battery is dead. You can keep cranking to get that one more cylinder over TDC and get it running… maybe. The P&W 1340 on the T-6’s has those kind. Starter need torque, either from battery amps, or hand cranking the flywheel. Really need to know what you are doing on the crank and in the cockpit. It is dangerous. )
And now we have just enough amps being delivered for the starter to do the job.
A curious type puts a volt meter across the starter terminals just as we do all this and it is showing17 volts… WTF??
Starter does not care about volts, just the amps. You could hook a 90 volt source to the starter and if it can only provide 10 amps, that is all it can provide. The volts don’t mean anything in a circuit like this. Amps/electron flow is the name of the game.
A starter motor is a direct short until it starts turning for all practical purposes. A battery can provide way bigger amps due to wire size then due to voltage. check your car when you hit the start circuit. Battery goes almost to nothing, it is making electrons as fast as it can and they are free to run through those big wires, does not need a lot of pressure ( volts ) to make them go. And amps are what the starter motor needs/wants
Yes, batteries have some internal resistance but if it is in good shape, there is no way it will be enough to stop or limit current in any meaningful way. Wire connections are a much bigger problem.
So shortest path to the starter is the negative path as that is the direction of the electron flow and you also want as little restriction for them to go all the way around.
So question me this or at least think about it.
You need amps, your 24 volt stuff does not quite get the job done so as you hit the starter connection and the voltage goes down to 8 volts on all batteries connected, would not adding another 12 volt source of amps add to the amp flow until the system voltage climbs back above 12 volts?
Now you can build an unusable system with a big high current load that must only use batteries and play around with just 3,12 volt batteries, attach voltmeters and amp meters everywhere and
The reading and how they change and which one changes the most/fastest/restricts flow the most and you may just learn a bit about it. But I will be curious as to what usable work will be developed for how long at what overall level and why it is thought to be a way to get things done?
Now, if this is too incoherent to understand, just say so… I won’t make it worse, I’ll just bow out.
I would be most interested an anybody who actually sets up the experiment and has the meters and such and can show that the one batter will go to not only 0 volts but will block or try to charge in reverse. Using wire, 3 12 volt auto batteries of like kind and a load that will drain the whole system without blowing up a battery or melting something so the circuit becomes open and every thing stops instead of going to the end and getting the results as is being claimed by those up thread.
Please use all safety equipment & procedures because I think there will definitely be fire or booms or flying battery parts. Acid is no joke. Be careful.
As said up thread
You are talking about a motor-starting application, and for this instance, you are probably right.
High currents, low depth-of-discharge.
But, the OP is talking about a (potentially) deep-discharge, solar PV system, which is where you get into problems as the batteries are discharged to the point of reverse-charging.
It’s not rocket science. Just think about what happens if you charge a lead-acid battery backwards (you ruin it), and also think about discharging a system of series and parallel batteries all the way to zero.