Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Design and function: Charging
The process during charging
- Negative plate: Lead sulphate is converted to pure lead
- Electrolyte: Water is converted to sulphuric acid
- Positive plate: Lead sulphate is converted to lead oxide
- The power supply from the generator or the external battery charger.
During charging, energy is supplied to the battery. This causes an electro-chemical process that is the reverse of the process during discharge. The lead sulphate (PbSO4 ) in the negative plate is converted back to pure porous lead (Pb) and the lead sulphate (PbSO4 ) in the positive plate is converted to lead dioxide (PbO2 ).
Water (H2 O) is consumed during the charging process. Sulphuric acid (H2 SO4 ) is formed. The density of the electrolyte increase as the amount of sulphuric acid increases.
For charging AGM-batteries, use only chargers that are both current and voltage-controlled. AGM-batteries are sensitive to overcharging and must be charged with an adapted charger. This since a battery that is charged with too high voltage/current does not absorb all the energy and the excess is converted to heat. When the battery becomes too warm the electrolyte evaporates (acid). When the pressure in the battery becomes too high, the gas is released through the battery box safety valve. When the water volume decreases the acid concentrates to an unacceptable high level, which may destroy the battery
AGM-batteries may be charged with a max. voltage/current as follows.
| Battery capacity (Ah) | Max. current (A) * | Max. voltage (V) | Max. charging time (h) ** |
| 100 Ah | 25 A | 14,4 V | 5-24 h |
| 90 Ah | 22,5 A | 14,4 V | 5-24 h |
| 80 Ah | 20 A | 14,4 V | 5-24 h |
| 70 Ah | 17,5 A | 14,4 V | 5-24 h |
| 60 Ah | 15 A | 14,4 V | 5-24 h |
| 50 Ah | 12,5 A | 14,4 V | 5-24 h |
| 40 Ah | 10 A | 14,4 V | 5-24 h |
| 30 Ah | 7,5 A | 14,4 V | 5-24 h |
| 20 Ah | 5 A | 14,4 V | 5-24 h |
| 10 Ah | 2,5 A | 14,4 V | 5-24 h |
| 5 Ah | 1,25 A | 14,4 V | 5-24 h |
* Max. current is calculated with the following formula (Battery capacity Ah/20)*5. For example, for a battery with capacity 70 Ah: (70/20)*5= 17.5 A.
** Charging time depends on how discharged the battery is, however, max. 24 h.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
Rather have a pro handle this?
A certified mobile mechanic comes to your home or office. They bring the tools, you stay comfortable. No tow truck, no waiting room, no surprises.
No obligation. See pricing before you book.