Lead acid battery construction
Lead acid batteries are a simple technology, and have changed little since the 1800s. Battery banks for offgrid use are also expensive, making home made battery banks an attractive option.
To make a lead acid cell requires only a glass or plastic container, lead roofing sheet (new or old stock, not used), 4M sulphuric acid, deionised water, petroleum jelly (eg vaseline) and some plastic to hold the lead plates in place. A hygrometer is used to achieve correct acid concentration.
Design features explained
Making life easy
Modern mass produced batteries feature multiple moulded plates with separators, lead compound pre-charge and antimony, but these are all optimisations rather than core features, and cells omitting these are much quicker to make, and work entirely satisfactorily. Simplifying the construction makes this a practical project.
Starting batteries, as used in cars, use interleaved lead plates to maximise cranking current. For off-grid use a much more durable plate layout is to use just 2 plates, widely separated. With this approach there is no need to connect multiple plates together or use separators.
The 2 electrodes are made of lead roofing sheet. Lead sheet can be rolled or folded to make an electrode with enough area. Just a little space should be left between the folds to allow ions to flow freely to all parts of the plates in use.
Spacing the 2 plates by 2" helps create a robust long lived cell. Even large amounts of plate distortion doesn't cause any problem. Folding the plates in such a way that each plate wraps around itself improves durability a little further by providing additional restraint against plate distortion. Eg:
_____ _____ | _ | | _ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |___| |___| |
The plates need to be suspended off the base of the container, because they gradually shed lead compounds which will short the cell otherwise. The gap between plates and bottom should be at least 0.25", and a generous 0.5" makes for a cell proof against even heavy shedding.
Plain lead plates only reach full electrical capacity after several charge cycles. These batteries however don't use plain lead, roofing lead has an oxidised surface, and experience so far is that they worked after one charge. The gradual increase in capacity with repeated charging is most easily addressed in an offgrid installation by simply putting them into service, where they can gradually get themselves upto full capacity without further attention.
The battery may need a first charge using a charger that delivers above the rated battery voltage. This can be a mains charger, or where mains isn't present a lower than 12v battery may be connected to a 12v system for its first charge. Never parallel cells or batteries when doing this!
Since the additional costs are minimal, a spare cell is handy to have. If one cell fails the spare can be wired in, avoiding long downtime. It should be stored dry to acheive indefinite storage life. Lead acid cells don't survive long if stored wet & not reguarly charged.
Everything that goes into the cell must be thoroughly clean. All internal parts need to be rinsed with deionised water before assembly. The lead roofing sheet is washed with tapwater, then rinsed off with deionised water before use.
Its recommended to make individual 2v cells rather than 12v devices in one container, as
- suitable containers are easier to come by
- construction is easier
- the battery can easily be reconfigured
- each cell can be checked or monitored individually
- a bad cell is easily replaced
- handling weight is kept much lower
- handling is easier
- less weight means much reduced risk of injury
The 2 electrodes are made of lead roofing sheet. Its cut to shape, washed & rinsed. Don't forget to leave a long tail on each electrode to enable wire connection to be made away from the acid bath. Capacity is in the region of 1Ah per one square inch of submerged anode and 1 square inch of submerged cathode.
Plastic bars or other suitable restraints hold the plates in place. This must be robust enough not to result in shorting during handling. Corrodable fixings (eg metal screws) must not be used. Plastic fixings are ok, such as plastic cable ties and plastic screws.
Plates should be suspended at least 1/4" above the base of the cell container, and preferably 1/2" to allow accumulation of lead compounds without causing shorting. These compounds are gradually shed from the plates in normal operation.
Wire connections to the lead plates should be made far enough away from the acid so as not to corrode from the inevitable acid spray. Connections and wire ends should be completely coated in petroleum jelly - don't use other types of grease for electrical work. Such metal needs to be arranged so that any copper corrosion products won't run into the cell. A simple way to achieve this is to cut a tail on the end of each plate, and bring the tail out of the top of the cell container and down the outside.
____________ | | | | | | | | | | | |__________|_| Cut the plate
_ | | | | | | __________| | | |_| | | | | | | |__________| Bend the tail upward
Battery containers need to not react with sulphuric acid, and not conduct electrically. Plastic and glass are good options. They must be strong enough to survive all handling without any risk of breakage. Containers need tops/lids to much reduce the gradual acid spray that occurs in use. Tops also make construction a bit easier and reduce the chance of spillage.
For end use, the entire battery bank should be enclosed in a durable secured container to keep debris out, prevent accidental touching of acid (eg by children) and avoid spillage. Batteries spray tiny amounts of acid in use, so should sit on something that can survive or neutralise this. The usual options are a lined wooden container or concrete.
Acid concentration is important, but not critical, and should be in the range marked as healthy on the hygrometer. The acid weakens slightly during the first charge, and acid is easier to dilute than strengthen, so the cells can be filled with acid toward the stronger end of the range initially. Acid concentration should be corrected after initial charge.
Lead acid battery construction involves working with sulphuric acid, which has significant health hazards. Sulphuric acid can eat eyeballs if mistreated, so if you don't know how to handle acids safely, try another project.
When charged, the cell plates form weakly attached lead compounds, some of which are shed over time to the bottom of the cell. Thus once charged, cells should only be disposed of either at suitable facilities, or in accordance with current legislative requirements.
Batteries should be kept in a secured container in use to avoid risk of spills and child access.
4M sulphuric acid has been bought from lab reagent suppliers and lead acid battery manufacturers. Vitriol could be used, but increases the risks involved.
Lead sheet is available at any builder's merchants or DIY shed.
Tupperware style plastic containers are available from kitchen goods retailers.
Deionised water is available from chemists and car accessory dealers. It can be harvested from dehumidifiers and a/c too, but this should be checked for conductivity, as contamination is possible.