Off Grid Power Systems III – Lead Acid Batteries

After discussing some properties of electricity and common methods to generate it, it is time to talk about how it can be stored. There are different types and designs of rechargeable batteries. This article is about deep cycle lead acid batteries. The principle difference between deep cycle lead acid batteries and those that are used in almost every car is the geometry of the electrodes. The former is designed to provide a small current over a long period (hours), while the latter to provide a large current for a short period (seconds).

Let me begin with my conclusions:

  • purchase the best batteries you can afford
  • purchase them after having bought and installed the rest of the set-up
  • if you don’t know much about them, do not buy second hand batteries
  • when purchasing new batteries check the production date stamp. Unused batteries are not necessarily like new batteries. Unused batteries must be kept at full charge.
  • storage, charging, discharging and maintenance are of paramount importance when it comes to efficiency and longevity
  • a battery bank should always consist of identical batteries. Mixing different batteries or same batteries with difference charge capacities is not the way to go
  • you can purchase the best lead acid batteries and destroy them very quickly by taking the she’ll be right attitude
  • yet, you can also get 5-10 years out of them without much decrease in capacity if you maintain them well
  • maintenance of lead acid batteries is very easy! They have been invented in 1859 and are still vastly around, mainly due to their simplicity and easy maintenance.
  • bigger is better. Ideally, you want to get the biggest one-cell lead acid battery you can afford and connect a series of them to operate at 12, 24 or 48 V DC
  • by doing so you can maintain each battery individually and when something goes wrong you just replace one cell

There is a lot of detailed information about lead acid batteries available online, for instance through wikipedia.

Three Types of Lead Acid Batteries

Before getting into some practical details, you must note that there are in principle three types of deep cycle lead acid batteries. Gel cell and amalgamated glass matt (AGM) types are both maintenance free. They can be used in any orientation and only these types can be stored inside. Since their performance is dependent on temperature, this can be an advantage in cold winters.

Standard deep cycle lead acid (DCLA) batteries must be regularly maintained, must be stored in well ventilated areas and orientated such that the electrodes face up. You should not lay them on their sides.

Basic Setup and Advice

DCLA batteries consist in principle out of two electrodes submerged into an aqueous solution of sulfuric acid (about 33.5 v%). Different, reversible chemical reactions occur during charging and discharging cycles and since chemists and physicists have different notations for what is defined as a positive or negative pole it doesn’t matter what I write here, some confusion is guaranteed.

All DCLA batteries contain solid lead, solid lead (IV) oxide, solid lead sulfate, dissolved lead ions and sulfuric acid. Lead dissolved in water is highly eco-toxic and highly toxic to many organisms such as human beings. There are two things to remember here:

The above mentioned solid forms of lead do not dissolve well in water, so you can touch the electrodes for instance and won’t come to any harm. Dissolved lead ions in water cannot be seen with the naked eye and it is this form which is highly eco-toxic. Don’t just dump the solution anywhere.

The sulfuric acid can also be quite harmful, should you get it in the eye. Further, during the charging cycle hydrogen and oxygen gas are formed and they will react with each other vehemently should you allow for the right conditions. In such conditions the battery will explode.

  • DCLA batteries that require maintenance should be kept in well ventilated areas
  • do not charge them when there is a naked flame around or when you are working with a grinder for instance
  • when checking the solvent levels be careful. Wear safety glasses or at least don’t stick your eye right on the battery. The classic mistake is to put your head right above the battery and think: ‘Hmm, it’s dark in there I can’t locate the solvent level, I’m gonna give the battery a kick and see what happens.’ Believe me if you don’t kick it, the guy standing next to you will kick it for you!

A 12 V DCLA battery consists of 6 individual cells, a 6 V one of 3 cells and a 2 V one of one cell. Ideally, you want to purchase 2 V DCLA batteries and wire 6 of them in series to obtain 12 V or 12 of them for 24 V. The great advantage is that when something goes wrong with a battery you only have to replace one (cell). Imagine a faulty single cell in a 12 V battery. Since you can’t just replace the cell, you will have to replace the entire battery. This can be expensive or just a huge hassle if it is the manufacturer’s fault. DCLA batteries are heavy and are classified as dangerous goods for shipment.

Voltages for common usages are mentioned in the wikipedia link. The general rule for discharging is that you should not put a load on DCLA batteries that results in a voltage less than 12.0 V.


Again, if you don’t want to have to regularly maintain your batteries, buy maintenance free ones. All DCLA batteries will have a stamp stating when this unit was produced, a sticker with some specifics and usually a ‘max’ and ‘min’ mark for the solvent level. Maintenance is easy, you will have to periodically open each individual cell, look into it and make sure that the solvent level height is correct. If your battery does not have these lines, the solvent level height should be just above the electrodes. Don’t fill the entire gap up!

When the solvent needs topping up, you will add pure water. Pure water is called de-ionized or distilled. Rain water will do the trick as well, unless you live somewhere with high air pollution, or collect your rain water in a cement tank.

Reticulated tab water should never be used, there are too many anions that will form non water-soluble lead salts that in turn adsorb onto the electrode plates and/or will precipitate to the bottom of the cell and eventually create a short. In this case, that cell is permanently damaged. You will spot this in many ways, but if you see DCLA batteries with curved side walls you can be sure that they are no good.

How ‘good’ are your batteries?

So far, the maintenance is pretty straight forward, isn’t it? Just add pure water to the required level. However, there is one more thing you need to do on  a regular basis. That is checking if the chemistry inside the DCLA batteries is optimal.

I could explain this exhaustively, but practically, all you need to do is buy a hydrometer. There are 3 other ways to check the state of your DCLA batteries:

  1. Using a hydrometer to measure the specific gravity
  2. Using a load tester, also called battery tester
  3. Using a voltmeter
  4. Using electronic gadgets like a battery monitor

The hydrometer is the ONLY correct way to check the state of DCLA batteries. I repeat, the ONLY correct way. The hydrometer is a syringe like plastic device. To check the state of your batteries, you will charge them up completely, allow them rest for a few hours, open each cell and submerge the hydrometer into the solvent, and press the balloon on its end to suck the solvent into the hydrometer. There is some sort of indication device (most likely a small red ball) inside the syringe and a scale. The scale is either in absolute numbers or indicates good, okay and bad.

If your hydrometer has a proper scale, it will tell you the ideal specific gravity of a fully charged DCLA battery, so you can compare the charge capacity of your battery as a function of time. In certain cases you can decrease the acidity in the battery permanently. Adding pure water will of course not concentrate the acid but further dilute it. In such cases you would need to get the acidity back to about 33.5 v%.

There are two rules when it comes to acids:

  • if you can’t express a concentration in volumetric percentage, g/l and mol/l, then stay away
  • when diluting a concentrated acid, add acid to the water not vice versa and do so slowly, diluting concentrated sulfuric acid is exothermic

However, if you purchase some special batteries from overseas, they are often sent out dry meaning that you have to fill them up appropriately.

Again, measuring the specific gravity is the only way to determine the (charge) state of (DCLA) lead acid batteries, every other method is inferior to this!

2. Using a load tester. This is a device with two electrodes and wires comparable in size to jumper cables. It has a scale on it, you read its manual, then wire it up to the battery. It will short the battery and give you a reading of available charge state. Quick and not messy.

3. Using a voltmeter. The potential difference between the electrodes which is measured in volts has almost nothing to do with the charge state of your batteries. If you measure the voltage and obtain 12.7 V, you might assume that your battery is in perfect order, but it is a mere assumption and nothing else!

If you need to check the state of a DCLA or a car battery and only have a voltmeter at hand, measure the voltage at rest, then start your car or apply a large load and monitor how the voltage drops. Example: rest voltage = 12. 7 V. You start the car and it drops below 12 V when the starting motor cranks the engine, then goes back to something like 12.4 V -> you need to buy a new battery soon. The voltage will not drop that significantly if the battery is in good nick.

4. Using battery monitors. Yes, they have heaps of information, charge state, voltage, current, energy and so on. But these measurements are all taken electronically and I suppose the monitor also needs to cache some information over time and extrapolate it. This means that some of the readings will not be as accurate as a specific gravity and that they might just be wrong should the monitor use some dodgy logic, which it probably does.

Practical and periodic use of DCLA batteries

General rule; never allow your batteries to operate at below 12.0 V. Remember, the electrical energy is stored in form of chemical energy in batteries and different chemical reactions occur during discharge and charge cycles. Lead sulfate is a white salt which hardly dissolves in water, it appears slightly greyish in batteries, but that will be a combinatoric effect. It dissolves well in sulfuric acid at (guess the concentration), however, its solubility decreases during the discharging cycle. This is because the concentration of the sulfuric acid (acidity) decreases during discharge.

Operating below 12.0 V is for this reason damaging to DCLA batteries. I need to mention here that during discharge, both electrodes produce lead sulfate which adsorbs onto them. During the charging process, the reverse reaction occurs, the acidity increases and the solid lead sulfate dissolves fully so that the electrodes should be lead acid sulfate free.

When the acidity is decreased too much, however, some of the lead sulfate will not go back into solution and will remain on the electrodes, reducing their conductivity. This occurs also ‘naturally’ in DCLA batteries other wise they would have a life time guarantee, but permanent sulfatization occurs much quicker when discharged heavily.

It will also precipitate to the bottom of the cell and this build up will short the two electrodes over time. The electrodes have huge surface areas – less in DCLA compared to car batteries – and they can only perform optimally when they are lead sulfate free when fully charged.

The longevity of your DCLA batteries will depend highly on how much you discharge them averagely and how often. 20% discharge and I would say you can use them for up to 10 years, 50 % is the maximum that manufacturers recommend. You must take this factor into account when calculating your energy requirements.

A 100 Ah DCLA battery can only provide in an ideal scenario 50 Ah. By ideal, I mean how quickly you discharge them, because this is yet another factor for longevity.

Charge controllers are utterly necessary, because you can do a lot of damage to the batteries and to yourself when over charging them. If it smells of rotten eggs near your batteries and there are no rotten eggs in the vicinity, your charge controller is malfunctioning and overcharging. The sulfur analogous of water (H2S) is being formed. This gas is eco-toxic, flammable and highly toxic to human beings.

Our body’s physiology plays a trick on us – well, more a warning than a trick really. Our sense of smell has a very high resolution, we can pick up comparably greatly diluted amounts (one part in a billion parts). The trick I mention is like this:

If you cannot smell H2S, it is either not there or your uptake of it is already alarmingly high, and this is why you can’t smell it any more. Soon you’ll fall over and die, so get out and get fresh air. Unlike carbon monoxide, fresh air will be your rescue.

Reviving DCLA batteries

Lead sulfate build up is the most common factor for poorly functioning DCLA batteries. While you can take measures to dissolve the lead sulfate (and get rid of it), you cannot change any permanent damage to the electrodes. Permanent damage occurs when they are short-circuited, and electrodes with lead sulfate build up tend to bend during heavy discharging.

Charge controllers ‘shake’ the solid lead acid off the plates, which should have dissolved, during a process called equalization. I believe this is done physically due to short pulsed strong currents. Chemically, lead sulfate can be dissolved best using a chelating agent like EDTA, perhaps household chemicals like citric acid, vitamin C and tartaric acid would do a good job, too.

But this is a messy job and commercialized DCLA batteries contain other metals in the electrodes and you would potentially change their ‘make up’.


As I mentioned, if DCLA batteries have an operational voltage of less than 12.0 V, turn the load off! It doesn’t matter if your batteries show 12.5 V without a load, if the voltage drops below 12.0 V during loading, you are discharging them too much. It doesn’t matter what the stickers on the battery say, it doesn’t matter if you calculated that you have enough energy in the batteries, it doesn’t matter what your Nanna says either…

This link takes you to the manufacturer’s specification web site for DCLA 6 V batteries (Deep Cycle Specifications). Amongst some important data like their weight (55 kg each) you’ll find the capacity in (Ah) and the so-called C-20, C-10 and C-5 values. C-20 gives you the maximum continuous discharge current for a period of 20 hours. Hmm, I’m baffled to why it states here 395 Ah C-20 and 320 Ah C-5 (instead of 285 Ah C-5).

Anyway, the calculations are self-explanatory, C-20 395 Ah means that you can draw 19.75 A for 20 hours, or about 120 W for 20 hours at 6 V (it will be less than 120 W due to resistance). Now, I would still say that these values need to by multiplied with 0.5 as you should only discharge them to 50% capacity, but to be sure you would need to look at more detailed specifications.

I have been ‘given’ two such deep cycle batteries, they are second hand and from another manufacturer but otherwise similar. Mine hold nowhere near 395 Ah though. I’d estimate they had practically 150 Ah C-20 when they were new and have now about 100 Ah. Unfortunately, there are no stickers on them in respect with charge capacity.

I think that is it, at some stage, if you find this useful and want more, I’ll write about charge controllers and DC-AC inverters, which are two more units that are needed for an Off Grid Power System.

2 thoughts on “Off Grid Power Systems III – Lead Acid Batteries”

  1. Yes this is useful, and yes I’d also like a reference work on the other components. I’m ultimately hoping to attempt an off grid set up here in the UK. Whilst it will not be as effective as something in NZ, if it can be done in Germany then it should be possible here. Apart from all my trees!

    1. Cheers Paul, I’ll write something up about charge controllers and inverters in the not-so-distant future. Solar panels are getting really affordable, about 250 NZD for 200 W at the mo. 10 of those and most households could produce their own power.

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