two 12 volt car batteries in series, weird charge voltage. - Page 1
Forum section: electronics / beginners

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 14, 2014, 10:41:13 pm
content: Not my setup. Basically in a bus. Apparently with the engine running, one battery is getting close to 16volts, the other is 12volts.Apparently the batteries are wired directly in series, and are matching, near new batteries.Why would this be happening? I assume one battery is bad, but which one and why?I believe that the low battery did have a load on it separate from the other battery, possibly throwing the batteries out of balance?

author: electr_peter
author_rank: Unknown
timestamp: May 14, 2014, 10:55:02 pm
content: Quote from: AmmoJammo on May 14, 2014, 10:41:13 pmInteresting setup.According to the description, I assume that 2 batteries are in series, alternator is trying to charge "double voltage" battery and the is some load accross one of the batteries. If the load on that single battery is significant, all alternator current goes to unloaded battery and only part of this current goes through loaded (while some current goes to load). This can explain voltage discrepancy between batteries (one battery is well charged or overcharged, other battery is only partly charged).I am not expert on car/truck/bus lead acid batteries and their reliability, but this setup very likely will have negative effect on one or both of the batteries due to uneven loading and charging. One battery could be significantly overcharged (really bad), other may be discharged too much [too little charge(bad), both batteries may not provide enough current for starter (bad)].

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 14, 2014, 10:58:54 pm
content: After 5 days, the bus won't start. Lol. Im pretty sure the batteries get put out of balance, and haven't gotten back into balance even after the load was removed, which was about 3 amps.I've instructed the guy to charge each battery off a 12volt charger, and he's now using a 24 to 12volt regulator to keep the load even across the batteries

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 14, 2014, 10:59:15 pm
content: Batteries are like the old saying about chains - a chain is as strong as the weakest link .A 12v battery is made up of 6 cells in series and is as strong as the weakest cell & then 2 12v in series is 12 cells , if just one of those cells is bad , out of water / electrolyte that the hole string is not going to work .When ever building a bank out of one or more batteries , you need to have the same make , model & manufacture date , to get the most out of them .Where are you measuring each battery ?16 volts , after the engine has been running for even just 15 min. is over charging , the 16v indicates to me this is a 12v system / parallel and not series , otherwise with the engine running you would see closer to 24v - 26v - charging .Some systems do use 24 volts , and some use 2 12v batteries in parallel .So I would have to guess you have something wrong , either in how you are measuring , or the series / parallel issue .To get a good test of this battery bank is to charge them separately and then test separately .

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 14, 2014, 11:01:53 pm
content: They're in series. One battery is at 12volt, the other at 16volt, with the engine running :/

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 14, 2014, 11:09:03 pm
content: Quote from: AmmoJammo on May 14, 2014, 11:01:53 pmThen you need to give more detail about this bus , what your saying does not make sense , 16v battery would have to have 8 cells .Or is one of them AMG battery , they have higher voltage .Back how batteries systems work , you can not have a mix of different batteries .Again if there are in series for a 24v system and the engine is running , you would have to have about 26v charging current .

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 14, 2014, 11:10:13 pm
content: Quite clearly, you don't get it, and should give up. Thanks for your input though.

author: David_AVD
author_rank: Super Contributor
timestamp: May 14, 2014, 11:26:39 pm
content: Sounds like it was a simple case of drawing a load from the "lower" battery. I can see why that would not be a good idea.

author: sleemanj
author_rank: Super Contributor
timestamp: May 14, 2014, 11:30:08 pm
content: Quote from: AmmoJammo on May 14, 2014, 10:58:54 pmWhich is why the starter battery should be separated from the house battery/ies :-)

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 14, 2014, 11:36:48 pm
content: I am not an electrical engineer , but have been in the auto repair bis for over 30 yrs. and worked on just about everything from scooters to heaving equipment .I gave you a description of what should be - 2 options 12v system & 24v system , and how you should treat & test batteries .You gave limited info , battery voltage readings should have 2 more digits in the description , as an example a 12v battery after charging and least to rest voltage should be 12.75 .I guess I am done .

author: turbo!
author_rank: Regular Contributor
timestamp: May 14, 2014, 11:49:54 pm
content: This happens inside a single battery pack too. Disconnect the batteries and connect them inparallelvery carefully and charge overnight using a 12v rated battery charged. Relocate the battery so they're in the opposite location. More than likely you have some loads that run off of the center and one of the ends and if the said load is left on, it will selectively drain one battery.You need to run larger 12v loads like fans, high power stereo, etc through a 24v to 12v converter to avoid uneven loading. Map lights and such that is used only a few minutes at a time with the engine off from the center joint is not an issue, but if you accidentally drain it, you need to do the above 12v restoration process.No two cells are equal. Say you have six 2v 30Ah cells in series. When you let it sit for 6 months and the remaining charge on all cells are about 75%, but one is at 50%. If that cell has sulfated, the impedance is higher.Suppose you let it charge to 15v. Ideally, each cell gets 2.5v, but say the bad cell has doubled in resistance. You'll still get 15v combined total, but the bad cell will have 4.28v across it while all the other cells only get 2.14v which prevents them from becoming fully charged without the bad cell lowering in resistance. This will cause the water level in bad cell to lower like nuts and it will need to get replenished.There is always some imbalance. Car batteries aren't that picky, but lithium-ion batteries are very intolerant of over voltage, so series wired cells always have an active bypass to limit voltage at each serial segment.

author: AG6QR
author_rank: Frequent Contributor
timestamp: May 15, 2014, 12:00:34 am
content: Quote from: KD0CAC John on May 14, 2014, 11:09:03 pmHe didn't say the battery produces 16v. He said it's got 16 volts across it while the engine is running. If so, it's being overcharged and won't last long.My guess, the engine's alternator/voltage regulator is trying to charge the pair at something just under 28V, which is reasonable, double the approximate 14V of a more conventional 12V system.The two batteries aren't balanced, so one takes more voltage than the other. Why are the batteries not balanced? Impossible to say, with the sparse information given, but a shorted cell on the battery with lower voltage is one possibility, a nearly-open cell on the battery with higher voltage is another possibility.If there was a load being drawn from the midpoint between the batteries, that could have caused the problem.It would be interesting to know the voltages when the engine is not running.My recommendation: First, replace the batteries, then connect the batteries directly in series. The negative of the "high" battery should be connected directly to the positive of the "low" battery, with nothing else connected to that midpoint of the batteries. If you need 12V, regulate it down from the 24V supply.

author: turbo!
author_rank: Regular Contributor
timestamp: May 15, 2014, 12:04:33 am
content: In the long run, an active charge balancer or a bipolar alternator(chassis, -12v, +12v) would be helpful if they're even available.A charge balancer simply imposes a load across the weaker battery to match the impedance of the stronger battery so the voltage between the center and each ends are equal.It wouldn't be able to absorb a huge level of disparity as the difference must be dissipated as heat.If the alternator is charging at 500W, each battery should get 250W. If the weak battery is only able to carry half the current, the resistor has to dissipate 125W.Before replacing the battery, I would do the parallel 12v charging first as I explained, reposition the battery, and observe. Once it becomes unbalanced, the fully charged battery will prevent the drained battery from fully charging, so, parallel equalizing is the first step to restoration attempt.If the problem follows the position, it is obviously caused by the load. This is like rotating tires. If the variation in wear is small enough, you could rotate it around. If wear difference is too big, you'll need to replace the more worn one and put the new one in the higher wear position.

author: turbo!
author_rank: Regular Contributor
timestamp: May 15, 2014, 12:26:00 am
content: After doing the parallel charge on a 12v charger, check the acid in each cell with a hydrometer if the battery isn't completely sealed.If you find one cell that is a lot lower than the rest, its a good chance the battery needs to be replaced.

author: SL4P
author_rank: Super Contributor
timestamp: May 15, 2014, 02:45:43 am
content: Quote from: KD0CAC John on May 14, 2014, 11:09:03 pmOh dear - give the man a match.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 15, 2014, 04:19:08 am
content: I instructed the guy to charge the batteries separately off a 12 volt charger, as I assumed they had been put out of balance by the unequal load. He's also now running the load off a voltage regulator, so fingers crossed it all works out! I just thought there may have been more to it that I've over lookedI guess it took 5 days for the batteries to get far enough out of balance that the bus wouldn't start.

author: Rick Law
author_rank: Super Contributor
timestamp: May 15, 2014, 05:30:56 am
content: Quote from: AmmoJammo on May 14, 2014, 10:41:13 pmAt 16V, your 12V battery odd to be bubbling by then. At room temperature, the gassing voltage will be around 14.5V.Just don't be smoking near it.

author: calexanian
author_rank: Super Contributor
timestamp: May 15, 2014, 05:34:04 am
content: We have this same problem in out new electric forklift. It uses 4 deep cycle batteries in a series parallel config for 24 volts. The proper way is either a battery intended for 24 volt use, or when charging have a scheme that charges each set at 12 volts. Two chargers, or a relay system or the like. Bad design.

author: ConKbot
author_rank: Super Contributor
timestamp: May 15, 2014, 06:44:40 pm
content: Sounds like someone is stealing 12v from one of the batteries to use to power a 12v load, and not putting it back when they are done so the batteries get unbalanced. On a 24v system with new, matched batteries, balancing should be easy/not problematic.A dual voltage system (+12 and +24v) should have a dual voltage alternator with a 12v and 24v output on the alternator so the batteries get charged properly and balanced, and the loads are powered from the alternator. A 24v to 12v regulator should work as long as it has enough current capacity to keep the lower 12v battery charged.Also, I'd suggest against connecting a fully charged and fully discharged lead acid starting batteries in parallel to charge them and equalize them. Sounds like a good way to have a bad time imo. 2 separate 12v chargers, one on each battery is the best way (or charging 1 at a time with the same charger)

author: SeanB
author_rank: Super Contributor
timestamp: May 15, 2014, 07:11:38 pm
content: If it is a bus there likely is a lot of spare room to have a dedicated 12V battery, alternator and idiot light to drive the 12V loads. Connect the idiot light to 12V using a 24V relay to switch it, from an engine derived supply like the fuel cut off valve, and then either have a switch to power the 12V loads or use the relay to connect them when the 24V rail is running. That way you can rn all the 12V stuff and if the battery is flat no worry about the bus not starting.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 15, 2014, 10:47:25 pm
content: Quote from: KD0CAC John on May 14, 2014, 11:36:48 pmexcellent!Anyway, This is a customer where I work at... I'll see if he comes back, and what he says when he does, although he's a regular, so I'm sure he'll be back some timeIt didn't help a whole lot that he had two guys telling him that having 12volts on one battery, and 16volts on the other, was perfectly normal in a series battery setup

author: deth502
author_rank: Frequent Contributor
timestamp: May 15, 2014, 11:56:01 pm
content: Quote from: AmmoJammo on May 14, 2014, 10:41:13 pm"basically" in a bus. so they are in a bus of they are not??so i skimmed through a few replies, about charging and what not, but the ADD is kicking n and im not feeling it, so ill just throw this out there, migh have been mentioned already, might not have, idk.USUALLY--- most of the time in a 24v system, everythign is still 12vdc, its just the starter and glow plug heaters tht run on 24v. the rest of the vehicle is std 24v stuff. so basically, one batter is just used for starting, while the other will take the brunt of the use/abuse. traditionally there will be 2 12v alternators, not a 24v alt. one charging each 12v battery. if a battery is charging at 16v, check for a bad regulator in that alt, or possibly a wiring screw up somewhere where someone got confused btween how to hook up series and parallel systems.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 16, 2014, 12:09:27 am
content: As far as I'm aware, the batteries are directly wired in series, and there is a single alternator.

author: Stonent
author_rank: Super Contributor
timestamp: May 16, 2014, 12:41:48 am
content: When my battery was having problems on my Jeep it was only reading about 10V. I charged it over night but it still wouldn't start, so I had to use the 55 AMP "Start" option on the charger and got it started. I had to drive about 45 minutes to the place that had the battery I needed. When I opened the hood, the inside was splattered with acid, the battery was swollen and steaming.I had to wrap it in a trash bag to pick it up and hand it to the recycler as my lead deposit for the replacement battery. Right before I handed it to him, I tested one more time with the volt meter, and was reading just over 8V so I had lost two cells and the alternator was pretty much boiling the acid inside and/or performing electrolysis on it. And yes there was a pretty rank rotten egg smell.So, was there any of that on the battery you worked with?

author: Nickk2057
author_rank: Regular Contributor
timestamp: May 16, 2014, 03:18:36 pm
content: Quote from: Stonent on May 16, 2014, 12:41:48 ammy battery was the same way :/ am lucky i took it back to the battery place to get it exchanged.. that thing could of caused a fire on my van D:

author: Mosaic
author_rank: Regular Contributor
timestamp: May 23, 2014, 11:56:12 pm
content: The OP needs a load pass circuit to manage the battery load balance and the charge currents.The 'normal' charge V on a 12V battery is around 14.2 to 14.4V. That way a bit of minor gassing happens to stir the electrolyte once the battery warms up.This also works against sulfation.The Teeter-Totter: Balancing a 28V+ alternator across two 12V+ batteries.Too much gassing as happens as 16V will corrode the positive grid and reduce battery life significantly. The 'undercharging' battery will sulfate and also die an early death.I attach a circuit that will tune in the load balance and solve the problem.Note the red & blue battery Voltage plots are now virtually identical with the active external 3A load on the Battery B. Batt A & B are represented as identical 2A current loads. The Alternator puts out 29V. Thus both batteries are charged at the same rate even with the unbalanced loading. When connecting : Batt B MUST be the one with the excess loading for this circuit to work. Both semiconductors should be heatsunk and kept away from engine direct heat. Mount low down in the engine bay for better cooling.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 24, 2014, 01:44:16 pm
content: Mosaic , now if we could a connection to each cell to keep them all balanced , or as close as possible .

author: G7PSK
author_rank: Super Contributor
timestamp: May 24, 2014, 02:05:30 pm
content: Its down to cell resistance, if there was no load I would say the one reading 16 volts had a higher resistance, but as there is unequal loading the one reading 12 volts while charging is due to the effective resistance being lower due to the load. The battery and its load are acting like 2 resistances in parallel the. You need to measure the voltage with the engine stopped, as the starting system is 24 volts when one battery is pulled too low the engine wont start.

author: Mosaic
author_rank: Regular Contributor
timestamp: May 24, 2014, 03:37:04 pm
content: The circuit shown effectively regulates the battery B voltage to about 14.4V, leaving the remaining alternator output of about the same 14.4V for battery A. It doesn't matter if the batteries are 'different' or have different loads/resistances. This is voltage based.Once the batteries are charged properly the vehicle will start. This unbalanced charging is the problem the OP stated. This circuit is about charge control not offline voltage regulation or loading.Changing the relative loads of the batteries to simulate batteries with different charge requirements simply requires adjusting the suggested 1K pot to retune the system.See the attached graph for a such a case. Note the red line shows the 4A thru the 2n3055.