# 3 12-Volt batteries in a 24volt system. **URL:** \ **Category:** Factual Questions\ **Created:** [August 31, 2016, 10:31pm UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610 "2016-08-31T22:31:41Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![BubbaDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bubbadog/32/333_2.png) [@BubbaDog](https://boards.straightdope.com/u/BubbaDog)\ **Post date:** [September 2, 2016, 12:25pm UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/21 "2016-09-02T12:25:45Z")
> [@GusNSpot](#): > > 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 All I got is a BSEE and 30 years experience as a power specialist. ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [September 2, 2016, 10:01pm UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/22 "2016-09-02T22:01:33Z")
> [@BubbaDog](#): > > All I got is a BSEE and 30 years experience as a power specialist. Bawahahahah, you win. Now show me a working system where you take batteries to a totally flat condition and expect them to recover. Seems **beowulff** thins you are talking about solar? A I want those batteries and B Why on earth do you need to add batteries? Improperly designed system? C Show me this system and tell me if it applies to all battery powered systems and which systems are so under-designed that it is even considered to place a single battery in a system that will always destroy it. Also. Explain to me how a bigger battery hooked to a smaller one, in series, both of the same voltage to make 24V out of two 12 V batteries get into a condition where the smaller gets electrons pushed through in reverse. There has to be another power source with enough voltage to reverse the batteries all together without needing to physically reversing the lower voltage battery. How do you get the current to reverse because it has to reverse in both batteries??? I need a lot more’ splains’ before I will think that you can reverse one battery hooked in series with another battery without reversing both. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 2, 2016, 11:00pm UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/23 "2016-09-02T23:00:57Z")
The current doesn’t reverse, it just charges the dead battery in reverse. In the battery, ions are flowing from one set of plates to another, changing one plate from Lead into Lead dioxide. Once the battery is completely discharged, continuing flow of current in this direction causes the opposite plate to be turned into Lead, which ends up damaging the battery. > **[Lead–acid battery](https://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery)** > > The lead–acid battery is a type of rechargeable battery first invented in 1859 by French physicist Gaston Planté. It is the first type of rechargeable battery ever created. Compared to modern rechargeable batteries, lead–acid batteries have relatively low energy density. Despite this, their ability to supply high surge currents means that the cells have a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in motor vehicles to provide th... ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 2, 2016, 11:10pm UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/24 "2016-09-02T23:10:45Z")
It’s been known for a long time that you shouldn’t put batteries in series that have different capacities. It’s bad during charging and it’s bad during discharging. (OTOH, you can usually get away with it when they’re in parallel. Just make sure they’re the same chemistry and at the same temperature.) I know others have already gone through this, but it looks like some people aren’t getting it: Let’s say battery A has high capacity (high A•hr rating), battery B has low capacity (low A•hr rating), they’re in series, and both are fully charged. As the batteries discharge, battery B will discharge at a faster rate than battery A. A point will be reached when the system (or person) that monitors the _total_ voltage (A + B) says, “Minimum voltage has been reached. Turn off load - do not discharge batteries any further. Time to recharge the batteries.” The problem is that, when this point is reach, battery B may be severely undercharged. This is very bad on battery B; you have caused irreversible harm to it and have shortened its life. The opposite happens when charging… you end up overcharging battery B, thus damaging it even more. Here is a link to a paper that discuss this very topic. The title of the paper is, “_Numerical simulation for the discharge behaviors of batteries in series and/or parallel-connected battery pack_.” The discussion on connecting batteries in series of different capacities begins in section 3.5. The authors say, > [@](#): > > The smaller-capacity battery [in the series circuit] remains at an over-discharged state during the rest of the system’s discharge process. When a battery over-discharges, its internal resistance increases, due to irreversibility. The battery shows definite **capacity decay** , **both performance and cycle life are defected**. In a series connection system, **failure of one battery by over-discharge seriously harms the system’s performance and safety** ; therefore **over-discharge must be avoided at all times**. In addition, safety precautions or power management should be put into design to protect both the batteries and the users. If a system of series-connection contains a battery of poor performance, the simulation program may predict when the system would encounter over-discharge, and helps to prevent the failure of batteries, extreme heat production, or any other danger situations. (Some emphasis added.) ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [September 3, 2016, 12:57am UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/25 "2016-09-03T00:57:02Z")
BTW, if you want to understand why the low-capacity battery gets charged in reverse think of it this way: (for this example, assume that the load is a 1Ω resistor, and that the internal resistance of the batteries is also 1Ω) 1. To start, both the high-capacity battery and the low-capacity battery are at 12V, and connected Plus to Minus. If you measure the voltage across each battery’s terminals, with the meter positive connected to the battery positive, you will read 12v on both batteries. 2. Now, you discharge both batteries until the low-capacity battery is dead. At the instant the low-capacity battery is completely discharged, the meter will read 0v across it’s terminals. 3. Immediately after that moment, you will find that the meter no longer reads 0V, but instead reads some non-zero **negative** voltage. How is this possible? Well, draw the circuit, except that instead of having two batteries in series, you have one battery, a resistor, and the load. The resistor is what the discharged battery becomes (it’s not a battery anymore, since it’s dead). Now, measure the voltage at different places in the circuit. Keeping the negative lead of the meter on the negative terminal of the good battery, there will be +12V on the positive terminal of the good battery, +12V on the positive terminal of the load, +6V on the negative terminal of the load, which means +6V on the negative terminal of the low-capacity, discharged battery. So, the low-capacity battery is being charged, **in reverse** , by +6V !!! +6V is being applied to the negative terminal, with the positive terminal at ground. That’s what the battery gets damaged. ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [September 3, 2016, 3:16am UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/26 "2016-09-03T03:16:30Z")
Charged? To normal people ‘charged’ means a useful thing. That battery is not useful in any way shape or form. By deliberately using a term that to most would mean one thing, although some might claim it was an innocent thing,m it is actually a deliberate move like trick bar bets and that is where it needs to stay. So this is not a circuit designed this way but incompetent people of higher education trying to do stupid things and then claim that through miss use of words, convince people who have to work in the real world that they have all the correct answers and a new way to get a 6V battery with useful power out of it. > [@](#): > > the big one still has plenty of juice, and it will force current through the lone battery in reverse, > [@](#): > > 3 12-Volt batteries in a 24volt system. > 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. Where does the OP say what kind of circuit it is? Okay, you want to say it is being charged in reverse, cool but without reversing the current you are pissing upwind IMO. And people bitch about my posts. ::: sheesh :::: And even if your multi-meter can show 6 volts after that battery is pulled out of the system ( I don’t think it will ) I’ll still bet you get no usable power out of that battery that you have charged to 6 volts. An explosion is more likely IMO. So I stand corrected, by using your logic you are 100% correct. And you know stuff that is not in the OP. > [@](#): > > 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. I still want to know where this info came from as even in re reading I can’t find it. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [September 3, 2016, 4:05am UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/27 "2016-09-03T04:05:01Z")
FTR, I didn’t think the type of system would matter, but it’s for a Humvee. The vehicle uses two 12 volt batteries in series to power a 24volt system. In addition to starting the vehicle, the system powers two military radios with power amps. On top of that, I’ve hooked up a 12 volt power converter to one of the batteries to provide 500 watts of AC power for charging phones and other radio batteries and stuff like that. I just want to be able to run the systems without the engine on for longer periods between vehicle starts without killing the batteries to the point where I need to jump it to start the truck. Since the 12 volt converter is only hooked up to one battery, I figured I would double the capacity of that battery by adding a third in parallel to it. Based on the answers here, I’m not going to do that anymore. I might, however, snag a 4th battery and go that route! Thanks again for all the answers! ---
**Author:** ![Ranger\_Jeff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranger_jeff/32/2065_2.png) [@Ranger\_Jeff](https://boards.straightdope.com/u/Ranger_Jeff)\ **Post date:** [September 3, 2016, 4:23am UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/28 "2016-09-03T04:23:53Z")
Maybe what you want to do is somehow rig it so although all 3(or 4) batteries take a charge from the alternator, but only 2 batteries are used to turn the starter motor. Use battery #3 (and #4) for the 12 v converter only. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [September 3, 2016, 5:05am UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/29 "2016-09-03T05:05:47Z")
Yea, if I could somehow keep a back-up power to use for starting the vehicle, that would be separate and undrained by other devices, it would be perfect. That probably takes a lot of extra wiring, though. ---
**Author:** ![BubbaDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bubbadog/32/333_2.png) [@BubbaDog](https://boards.straightdope.com/u/BubbaDog)\ **Post date:** [September 3, 2016, 11:21pm UTC](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610/30 "2016-09-03T23:21:35Z")
> [@GusNSpot](#): > > Bawahahahah, you win. > > Now show me a working system where you take batteries to a totally flat condition and expect them to recover. Seems **beowulff** thins you are talking about solar? > > A I want those batteries and > B Why on earth do you need to add batteries? Improperly designed system? > C Show me this system and tell me if it applies to all battery powered systems and which systems are so under-designed that it is even considered to place a single battery in a system that will always destroy it. > > … Are you addressing this to me? Because My posts didn’t address charging since it wasn’t in the op… I explained what happens when a high capacity battery (in this case two batteries in parallel) are placed in series with a lower capacity battery. The op wanted yo know if there’s a benefit to this. There is no benefit and somewhat likely that it may be detrimental. Look. People do this all the time, most typically in flashlights. The batteries get very weak in a 3 cell light and you only have 2 spares. So you put them in with one of the weak cells and carry on. Except the light dies long before the two new cells go dead. It’s the weaker (low capacity) cell being reused that halts the show. [Previous page](https://boards.straightdope.com/t/3-12-volt-batteries-in-a-24volt-system/764610.md?page=1)