Assemble a lead-acid cell by stacking formed plates alternately with a separator between each pair, beginning and ending with a negative plate, in a case with clearance at the sides and 20 to 30 mm of empty space below the plates for shed paste. Test that the positive and negative groups are not touching before any acid goes in, then fill until the electrolyte stands 10 to 15 mm above the plate tops.
Part of Battery Building and Maintenance
Putting formed plates, separators and electrolyte into a case to make a working cell, and checking that it is not shorted.
The Stack Must Keep Plates Apart And Leave Room For Sediment
Cells shed a little active material as they cycle, and it falls to the bottom of the case. If the plates stood on the floor of the case, that sediment would build up between positive and negative plates and short them. The carrier’s remedy is to leave 20 to 30 mm of empty space under the plates and 10 mm clearance on all other sides, and to hold the stack off the floor on ribs or a ledge.
Stack order: negative, separator, positive, separator, negative, and so on, ending with a negative plate, so a cell has one more negative plate than positive (Plate Manufacturing). The carrier’s 50 Ah example is 4 positive and 5 negative plates. Join all positive lugs with one lead strap and all negative lugs with another (Terminal and Bus Bar Connections).
What You Need
- Formed positive and negative plates — Plate Curing and Formation
- Separators — Separator Materials
- A case — Battery Case Construction
- Electrolyte at working strength — Electrolyte Preparation
- A continuity tester or ohmmeter — Continuity Test
- Eye protection and gloves
Method: Build And Fill A Cell
Step 1: Assemble the stack dry, with a separator between every pair and none missing. Cut separators slightly larger than the plates so no edge touches.
Step 2: Check for shorts. Before adding acid, test between the positive strap and the negative strap with the continuity tester. A pair of groups that is correctly separated reads open. A reading means two plates are touching somewhere; find it and fix it.
Step 3: Lower the stack into the case on its ledge, with space under it.
Step 4: Fill slowly with electrolyte until it stands 10 to 15 mm above the tops of the plates (carrier). Pour in a thin stream so as not to wash paste off.
Step 5: Check the cell. Read the open-circuit voltage, which should be near the figure for the acid strength (Specific Gravity Control). A cell that reads zero or very low has a short.
Step 6: Give it a first charge at a low rate and measure capacity with a discharge test to 1.75 V per cell (Constant Current Charging).
Worked Example: Internal Size Of A Case
All separator and stack dimensions here are assumed for the example, except where the carrier is cited. Nine plates (4 positive and 5 negative) each 6 to 8 mm thick when pasted (carrier) stack to 9 × 6 = 54 mm to 9 × 8 = 72 mm. Add eight separators at an assumed 2 mm each: 16 mm, giving 70 to 88 mm. Add 10 mm clearance on each side (carrier): 70 + 20 = 90 mm to 88 + 20 = 108 mm of internal width.
For height, take plates 120 to 180 mm tall (carrier), add 20 to 30 mm of sediment space below, and 10 to 15 mm of electrolyte over the plate tops: 120 + 20 + 10 = 150 mm at the minimum and 180 + 30 + 15 = 225 mm at the maximum, before any headroom for the straps and lugs.
Common Mistakes
- Plates resting on the case floor — sediment bridges them and the cell shorts
- A separator slipped aside — the plates touch and the cell reads dead
- Over-filling to the brim — gassing then pushes acid out of the cell (Water Level Maintenance says to fill to just above the plates)
- Leaving formed negative plates to dry in air — general knowledge, not from the other articles: sponge lead reacts with air and heats up, so keep formed plates wet and assemble promptly
Safety
A finished cell can short in an instant and then vent hydrogen
A tool, a loose strap or a lead wire laid across the terminals shorts the cell; the energy-storage article warns the short can weld the tool to the terminals and cause an explosion. Remove rings and watches, use insulated tools, and keep nothing metal over open cells. The first charge produces hydrogen, which collects at the top of any enclosed space and is ignited by a spark from a switch or a loose connection (Hydrogen Gas Hazard). The electrolyte burns skin and eyes (Acid Handling).
What’s Next
- Learn next: Separator Materials — what goes between the plates
- Combine with: Charging Circuits — bringing the new cell to full charge