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I have never heard of a lead acid battery having any electronics to balance it's cells, or even any manual way to balance them.

But I think all lithium batteries have, or at least lithium batteries above a certain size.

Why is there this difference between lithium and lead acid batteries?

Let's say I take four 12V batteries and put in series. Would I ever need, or would it be beneficial, to have any balancing, or can I just pretend it is one 48V battery?

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6 Answers 6

up vote 7 down vote accepted

The answers provided so far are a little light on the actual mechanics that warrant balancing on Lithium chemistries and not on others.

First of all; all battery chemistries benefit greatly from proper balancing. Balancers are used on spacecraft nickel cadmium batteries, certain types of (low discharge) lead acid batteries and so on. All battery chemistries are just a certain dominant chemical reduction-oxidation reaction which occurs between certain Gibbs energies (or Redox potentials if you take into account both the anode and cathode reactions) - hence between a certain lower and higher voltage level. Above or below this 'ideal' range of voltages, other reactions may occur - or otherwise minority reactions become dominant.

These other reactions often are not reversible, hence they reduce the amount of 'useful' anode and cathode material, reducing capacity. Sometimes such unwanted reactions are even more dramatic, creating compounds that corrode the electrodes, degrade the electrolyte or cause toxic/explosive chemicals to form.

Now, these dangerous reactions are the primary reason why lithium chemistries really require safety circuits. Both when overcharging and overdischarging, depending on the electrolyte used, an explosive gas mixture is formed. More importantly, when the anode becomes too hot (about 125C), an exothermic reaction starts which accelerates itself, consuming most of the energy stored in the battery (thermal runaway). This is often caused by self-heating when dealing with large discharge currents, or with unwanted reactions caused by overcharging. As lithium chemistry batteries have energy densities up to more than an order of magnitude more than nickel and lead chemistries, i.e. a lot of energy in a small place, this can cause a big boom. Especially when combined with an explosive hydrogen-oxygen atmosphere.

Other chemistries have the same problem, though! Wet-cell Lead acid batteries are very well-known for producing hydrogen gas, even in 'normal' use, but mostly when abusing the cells. Lead acid cells can also go into thermal runaway when the sulphuric acid is concentrated enough. However, because of the relatively low energy density and high thermal capacity of the plates, as well as the high temperature at which thermal runaway kicks in compared to lithium ion, this is not a risk that needs to be dealt with in most situations. And the same goes for nickel chemistries, which often come with balancers in high-current applications (e.g. RC cars) - or your battery will only last 10-50 charges.

Then there's the practical question: can you just put lots of cells in series and pretend it's one big high voltage cell? Yes, you can, but the battery lifetime will be horrible. Any cell mismatch in your 12-cell stack will be exacerbated each charge-discharge cycle, and after a couple tens or maybe 100 charge cycles you will have a dead battery. It may even cause a safety hazard. So both for your safety and optimal use of the batteries it is very strongly recommended to use balanced charge management.

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What I don't understand is: Why is two 12V batteries any different from a 24V battery? Both are just 12 cells in series, 12 cells from the same factory, all produced in the same way, aiming for the same specifications. – Mads Skjern Apr 21 '14 at 18:42
@MadsSkjern aiming for, but not necessarily achieving, the same specifications. Those differences may be minor when the batteries are brand new, but tend to drift further and further apart as the batteries age. – Grant Apr 22 '14 at 1:54
So with a 24V battery, they have done a great effort to make all cells identical, an even greater effort than they do in batteries with fewer cells? If both cells in 12V and cells in 24V are taken from the same 'population', then the variance between the 12 cells is the same with the 24V and the 2x12V. Right? – Mads Skjern Apr 22 '14 at 8:03
No effort is done to do cell matching or cherry picking for consumer grade batteries, but also fairly importantly: series strings of batteries have this positive feedback mechanism of any small mismatch in capacity being exaggerated with each charge cycle. This is because the 'weakest' cells in a pack are discharged relatively more (because they already have less capacity) and thus stressed more, leading to faster wear. So even if you do incredibly good cell matching, after a while they will still become imbalanced. – user36129 Apr 22 '14 at 9:06

Cells vary from battery to battery more than multi-cell mismatch in same construction. Hence 6 cell lead acid are treated as one. Since aging and capacity of unit is accelerated by weakest cell being depleted first, it is more critical with Lithium to optimize matching to improve overall capacity and prevent overcharging of weakest cell. Active zener clamp on each cell is needed to prevent overcharge.

However lead acid batteries will fail on 1 cell first more often than all degrade the same, but cost effectiveness does not warrant this added cost to extend life.

Also since self-heating accelerates aging in Lithium, they prefer fast charge and cutoff rather than rapid charge and CV charge of 14.2 of lead acid for float. SLA's are similar but lower voltage that is also temp compensated.

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Lead acid batteries are OK with a certain float charge current forever. Lithium batteries would be damaged that way. When a lithium battery is full, trying to charge it more will cause damage. Conversely, in a car the "12 V" lead-acid battery is usually just charged with a fixed voltage of about 13.6 V. At that voltage it will take a small amount of charge current even when full, but unlike with a lithium battery, this does the lead-acid battery no harm.

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So float charge will even out the cells. So if I connect four 12V batteries in series, the same principle applies, and each of the 24 cells will be balanced by the float charge. Am I right? – Mads Skjern Apr 21 '14 at 13:34
Float charge will even out, but rapid charge requires individual chargers for extended life of all 4 batteries, when 1 dies all should be replaced or 1 replaced with similar age. – user38637 Apr 21 '14 at 13:57
Could you elaborate what you mean by rapid charge? What if I have solar panels for charging the batteries, and they don't have a high power. Does rapid charge refer to anything more than float charge, or only to charging the batteries above some current/power? – Mads Skjern Apr 21 '14 at 15:26
Whoever downvoted this, it would be useful to explain what exactly you think is wrong. – Olin Lathrop Apr 21 '14 at 19:08
@OlinLathrop I did not down vote, though retract my up vote after reading "this (overcharging) does the lead-acid battery no harm". There is no evidence supplied that it does no harm. A less absolute phrasing like "less harm". Or lead acid battery chemistries have much larger overcharge tolerance then lithium batteries (0.10V on 4.2V for LiCoO2). The liquid lead acid battery tolerance goes into increased gassing resulting at least in water loss. Which can be partly "repaired" by refilling the battery with de-mineralized/distilled water. – Pro Backup Mar 14 at 16:09

In my experience pure solar remote area power works very well with lead acid batteries because: Charge rates are low for extended periods. The battery gets cycled slowly over a wide range summer to winter. There is excess energy relatively frequently to equalise at low currents

Diesel solar battery hybrid systems have: Much larger daily cycles. Rarely have energy available for low current equalization. Go from high charge to significant discharge without significant float time.

This confirms what user 38367 mentions, that individual cell balancing would be beneficial for lead acid batteries in such remote area hybrid power systems using lead acid batteries.

There is significant evidence that long string lead acid batteries rapidly lose capacity compared with similar cycles on a single cell.

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Because if you overcharge a lithium cell, it burns or explodes; if you overcharge a Pb cell, it just vents hydrogen until it gets dry; then you'll have just to add water to fill it up again. I think modern Silicon and AGM Pb batteries do not even vent, but rather they recombine hydrogen and oxygen into water; but energy in eccess must go somewhere, so maybe they get a little warm, don't know.

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Lead-Acid batteries ARE balance charged using a process known as "Equalization." The cells in the series string that have the highest charge are allow to be over-charged, and this in turn allows the lower cells in the string to fully charge as well.

Li-ion cells can't use this process because they cannot tolerate being over charged, so they need a active balanced charging process to get the same effect; all cells having the same charge level.

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