Who has done this and was it a success or a failure?

A while go I was given lots of unwanted and unused 12v 7ah lead acid AGM batteries. I wish to turn the into a 36 volt 21ah deep cycle battery pack, for a hub motor, using an arrangement of 3 batteries in series and 3 of these in parallel making a total of 9 batteries. I will point out that I would like the current to be drained evenly across them all rather than draining one set of 3 in series at a time. I have read up about using batteries in parallel and have found a lot of conflicting information.

I have read that batteries in parallel will damage each other and the power will drain from them unevenly, often leaving some fully charged and others are totally drained.

I have also read that it is good to put them in parallel as it causes less strain on each and as a result of resistance they will drain evenly as most of the current will be drawn from the ones with the lowest resistance which occurs when they have more charge.

And quite a lot more including some things that sound utterly mad.

I suspect that it is safe to use them in parallel and they would drain evenly based on internal resistance: however if left the ones with a slightly higher voltage will boost the voltage of the others. If so could be solved by having 3 switches to isolate them when not in use. Am I correct?

One thing I am unsure how to approch is recharging them. Would I be able to charge them in parallel with a single charger?

Am I likely to get any other problems?

Just a hypothetical curiosity that I have been pondering for some time that may be mildly relevant. If for example you power a motor with 2 voltage sources, in parallel with say unlimited current availability. But one voltage source is 12 volts and the other is 8. How would power be drained from the two voltage sources by the motor?

 

The 'trick' employed to make useable batteries and circuits using them is to overengineer the battery. DO NOT use the full amp hours capacity of a battery. If it says 7 then plan on using 3-5 Ah of the battery in your design.

This will get you past most of the drawbacks you have listed.

 

Brainfog,

Let's answer your hypothetical first because it has relevance for the rest of this answer.

If you wish to use a 12.0 Volt battery and a 8.0 Volt (strange voltage for a battery) in parallel and the batteries are isolated then upon turning on the switch the 12.0 Volt battery will try to charge the 8.0 Volt battery and try to supply the voltage(current) for the motor. The voltage across the 12.0 Volt will drop drastically and the motor won't come on.

If the batteries aren't isolated then the 12.0 Volt battery will try and charge the 8.0 Volt battery and damage the 8.0 Volt battery if 8.0 Volts is its' actual working voltage.

Bearing that in mind this is how I would setup your system. I would place 3 batteries first in series and then place 3 series batteries in parallel. The output of each of the 3 batteries in series would have a high current diode with a heat sink going to the motor. This stops the higher voltage of the 3 series batteries, that are in parallel from trying to charge the lower voltage of the 3 batteries that are in series, as in the 12.0 Volt, 8.0 Volt example.

As for charging the batteries make a special harness with 9 spade connectors per polarity for + and - for each battery to charge all batteries in parallel, therefore they won't be overcharged or damaged which can happen if you have a weak battery and a strong battery in series.

Of course, when you are charging the batteries you will make sure that the high current diodes are disconnected. To test the status of each 12.0 Volt battery after they are charged place a 2.0 Ampere (or higher, a car headlight, for the load and measure the battery voltage after 5 minutes)

If you are dealing with 9 batteries you may have to do a bit of a juggling act and switch around the batteries in their series/parallel combination.

Place in series in order of the highest voltage readings after their test the following grouping combination: - 1, 2, 3, then 4, 5, 6 and lastly 7, 8, 9
The arrangement should look like this:

+ + +
1 4 7
2 5 8
3 6 9
- - -

A weak battery in series with 2 good batteries can reduce the output series voltage of the other 2 good voltage batteries so we try and isolate the weakest voltages.

Danny

 

Were these batteries new when you were given them? If not then I might doubt their ability to perform the arduous task you plan for them. If each string of 3 batteries is not very very close there will be issues, perhaps minimized by the high current diode. The battery test suggested by "WellGrounded" should be the first thing you do in order to determine the 3 equal sets of 3 series batteries. Each battery should be charged individually for this test.

Aside from that, I feel the whole setup really is shaky at best. It's a sure way to wreck all the batteries faster than if they were used separately. But you did get them for nothing, so I suppose you have nothing to lose! It would be the last thing I'd do if I had just purchased the 9 batteries.

 

I would give each battery a conditioning charge, something like holding the full charge V on for a half hour after full charge is indicated; then would go to series parallel arrangement for use & charging.
Speaking of 8 V beeing strange, " we" usid to use two banks of 14v batteries to power a dynamotor & 144 vacuum tubes in a seismograph recording truck. A 6V battery with external strapping was split into 3 2V batteries.

 

Thank you for your replies.

I will start off by clarifying something in my hypothetical. How would it be affected if each voltage source had a diode so current could not pass from one to the other. This question was more related to how the motor would consume the current from the voltages. Don't worry I would not intentionally try to set up batteries in this way.

If I had bought the batteries I would have simply bought 3 higher Ah batteries instead. I will also note that the hub motor can easily run off just 3 of these batteries in series but for not for as long as I wish them to. The goal is to put less strain on each battery and in turn for them to last longer. I believe the hub motor is 150w however that could refer to rotational power or power being drained from the battery. Is this lower current low enough to bypass many of the problems that may occur?

I did consider, at first, using low drop out diodes to provide isolation at one point until I thought about how I intended on charging them. It is a good idea if I can get it right.

A little more info on the batteries: Of the 12 I have been given of which when they arrived were still in their original, upopened, box I found through testing that 10 were in good condition at about 13v. One was weak and another was in very poor condition which must have had high self discharge rate. The ones that are good I have been desulfating as they were sitting around for a little while to get them upto full capacity. This includes draining and recharging them to prevent lead dendrites (sp). Oh also all of the batteries are all from the same batch.

As for the juggling act of series/parallel it may not be worth the effort involved on a regular basis. If all the batteries are near identical voltage. Would I be able to get away with charging and discharging them in series-parallel if say I have a switch but instead of turning them on and off it simply switches which way round a heatsinked diode is connected?

Other ideas that I have include: setting them all up in series and using a switching regulator to bring the voltage down to 36v. Although I am worried about what would happen if this failed. Or building some kind of control circuit that could control the current drain from each battery. Such as something that rapidly switches which battery is drained using MOSFETs so each one is say used for 1/3 of a second each second. Assuming this wont damage the motors control system.

 

What are the odds of them actually being drained evenly even if they are all in good condition and isolated by diodes? And if not why?

Also is there any viability in my switching idea or some kind of regulator or controller being used?

Thanks

 

I'm not sure what output voltage you need. if it's 36V, you will be lucky to get that. As soon as you begin to draw current the operating voltage will quickly drop to 12.7V or lower.
12.7V X 3 = 38.4V. But remember you have just connected 6 diodes that have their own voltage drop of about .7V. .7 X 6 = 4.2V and 38.4V - 4.2V = 34.2V.

The switching idea could work to your advantage for charging, but I seem like an awful lot of work and time charging them.

 

Since you will be subjecting the batteries to cyclic use, they should be charged by a current-limited source to a rate of 1.5A-1.8A per battery, up to 14.2v-14.4v across the battery terminals at 25°C/77°F for the "bulk" and "absorption" charge phases.

After that terminal voltage has been reached, the voltage should be reduced to 13.4v-13.8v, limited to ~700mA @ 25°C for the "float charge" phase.

If the batteries are all performing identically, charging the whole pack in series/parallel theoretically won't make much of a difference.

I recently discarded a pair of APC 1000vA UPS'es that used four 7AH 12V sla batteries that had them wired two in series, then the two series strings wired in parallel.

Where you will get into trouble with a 3x3 array is when one of the batteries in the pack develops a shorted cell or two. This will result in the other good batteries in series with it to become overcharged during float by approximately 1.14v each, and in the other two strings, the batteries will be undercharged by about 0.76v each.

The batteries being overcharged will rapidly fail due to gassing; the electrolyte will be converted to oxygen and hydrogen, leaving the cells dry.

If the charger is on a timer to save on electricity, the one battery with the shorted cell will cause the other two strings of batteries to become discharged to around 50% capacity, which will also cause the batteries to accumulate plate sulfation.

If the charger is not on a timer, the shorted cell will cause much high power consumption than normal.

Using diodes in series with the batteries will basically accomplish nothing but waste power.

As far as regulating the output voltage - that's basically what a motor controller will do. There is no sense in using a regulator AND a motor controller, as the losses will be too great.

 

Where you will get into trouble with a 3x3 array is when one of the batteries in the pack develops a shorted cell or two. This will result in the other good batteries in series with it to become overcharged during float by approximately 1.14v each, and in the other two strings, the batteries will be undercharged by about 0.76v each.

Hopefully not a hijacking question. I have a 12V AGM battery, 35AH that when a decent load is applied the voltage instantly drops to between 10.5V and 11.5V. It charges fine and maintains it's charge O.K. as well but dies when asked to perform. Is this a characteristic sigh of a shorted cell? Been reading a lot at "Battery University" but haven't yet come across an answer.

 

You most likely have a cell where most of the plates have pretty much separated from the internal jumpers. That could happen due to excessively loading it, mechanical damage by shock/vibration, plates falling apart due to age or excessive discharge cycles, or a combination.

 

Hopefully not a hijacking question. I have a 12V AGM battery, 35AH that when a decent load is applied the voltage instantly drops to between 10.5V and 11.5V. It charges fine and maintains it's charge O.K. as well but dies when asked to perform. Is this a characteristic sigh of a shorted cell? Been reading a lot at "Battery University" but haven't yet come across an answer.

Feel free to ask a question.

I am no expert but to me it sounds like it is very high internal resistance. If I remember correctly you have a desulfator thread around and a desulfator may help if it is due to sulfation. What is its voltage with 0 load and how has it been treated in the past?


SgtWookie I understand how when charging with a shorted cell would overvolt the remaining cells in the string; but, I fail to see how that would affect the voltage in its neighbouring strings?

After contemplating my project I have decided that it simply isn't worth the effort and potential problems. What I will do is wire them to the same connector but use a switch to switch between them. I was hoping to boost their lifespan by lowering load on each one, but, I seem to have found many fascinating ways to kill them. Maybe a good project for the ones that are in not such good condition.

On a side note I am intending on attaching a small volt meter as a means of checking the remaining power, which will mean that a shorted cell will be more easily detected.

 

Feel free to ask a question.

I am no expert but to me it sounds like it is very high internal resistance. If I remember correctly you have a desulfator thread around and a desulfator may help if it is due to sulfation. What is its voltage with 0 load and how has it been treated in the past?

The battery is not that old, 2 years, and has been use for my scooter which was generally charges after each use. I may have cause a direct short at one time or connected the charger clips backwards, but I believe I have reverse protection with the chargers. I would find it hard to believe it would be sulfation.

SgtWookie I understand how when charging with a shorted cell would overvolt the remaining cells in the string; but, I fail to see how that would affect the voltage in its neighbouring strings?

After contemplating my project I have decided that it simply isn't worth the effort and potential problems. What I will do is wire them to the same connector but use a switch to switch between them. I was hoping to boost their lifespan by lowering load on each one, but, I seem to have found many fascinating ways to kill them. Maybe a good project for the ones that are in not such good condition.

On a side note I am intending on attaching a small volt meter as a means of checking the remaining power, which will mean that a shorted cell will be more easily detected.

I think that you switching scheme would be the best of all worlds as you could be charging a set while another is in operation, thus no long wait time after all 9 batteries have been depleted.

 

SgtWookie I understand how when charging with a shorted cell would overvolt the remaining cells in the string; but, I fail to see how that would affect the voltage in its neighbouring strings?

Once the charging ceases in a healthy battery bank, the individual 12v batteries will settle down to ~12.8v per battery. However, in the battery bank where one of the batteries has a shorted cell, that individual battery will settle down to around 10.6v-10.7v.

If you just had two batteries in parallel, one good, one with a shorted cell, the voltage level would try to equalize. This would discharge the good battery, and in the battery with a shorted cells, the good cells would be overcharged.

After contemplating my project I have decided that it simply isn't worth the effort and potential problems. What I will do is wire them to the same connector but use a switch to switch between them. I was hoping to boost their lifespan by lowering load on each one, but, I seem to have found many fascinating ways to kill them.

Placing a heavy load on them will not only discharge them very quickly, but greatly increase the chemical activity and raise the core temperature of the batteries, which would cause them to age at a more rapid pace.

The heavier the load on a battery, the greater the power that is dissipated within the battery itself due to internal resistance. In extreme cases, such as a dead short across the terminals, this can cause batteries to rupture or explode.

So what I suggest that you do is to operate the three strings of three batteries in parallel, but charge the three strings individually. That way if there is a problem with a string, you will be able to detect it easily.

Maybe a good project for the ones that are in not such good condition.

If the batteries are scrap to begin with, your results will be inconclusive.

On a side note I am intending on attaching a small volt meter as a means of checking the remaining power, which will mean that a shorted cell will be more easily detected.

That's why I suggest you charge the strings individually. If all the strings are hard-wired in parallel, it will be very difficult to tell which string has a problem. However, if they are electrically isolated, it will be relatively easy to tell.