How to size a battery charger
Most wrong orders are a sizing problem, not a quality problem. Four numbers decide everything: system voltage, battery amp-hour capacity, chemistry, and how fast you need to be back on the road. Work through them in this order and the model picks itself.
Step 1 — Confirm the system voltage
Count the cells or read the label: 6 cells = 12 V, 12 cells = 24 V, 24 cells = 48 V. Then confirm with a meter, because a partly discharged 48 V pack can read like a healthy 36 V one to an untrained eye.
Chargers are matched to nominal voltage, not to what the pack reads at rest. We stock 6, 8, 12, 24, 32, 36, 42, 48, 72, 96, 120, 144 and 156 volt units — the odd voltages (8 V golf cart, 72 V EV) are exactly where generic chargers stop working.
Step 2 — Choose amps from amp-hours
The rule of thumb across most manufacturers is 10–25% of amp-hour capacity. A 10% rate is gentle and maximizes life; 25% is the practical ceiling for a fast turnaround.
| Battery capacity | Gentle (10%) | Typical (25%) | Approx. recharge from 50% |
|---|---|---|---|
| 35 Ah | 3.5 A | 8.75 A | 2–4 h |
| 100 Ah | 10 A | 25 A | 3–5 h |
| 200 Ah | 20 A | 50 A | 4–7 h |
| 400 Ah | 40 A | 100 A | 5–8 h |
Bigger is not better. Oversized chargers shorten battery life and heat the pack; undersized chargers never complete absorption and leave the battery chronically partial, which invites sulphation.
Step 3 — Match the charge profile
Flooded, AGM, gel and lithium want different voltages. A gel battery charged on a flooded profile will bubble its electrolyte and die. Most of our chargers are selectable; the ones that are not state clearly which chemistry they serve.
If you run a mixed fleet, a selectable unit (Schauer, Quick Charge Select-A-Charge, Dual Pro) is worth the extra money — it replaces three dedicated chargers.
Step 4 — Connector, mounting, environment
- Connector: alligator clips for bench use; Anderson SB50 (blue) is standard on 48 V systems; 3-pin XLR covers roughly 90% of wheelchairs and scooters; golf carts vary by brand.
- Mounting: on-board units need flanges and an enclosed case; portable units want a handle and a cord that survives being dragged.
- Environment: marine and lift applications want potted or waterproof housings; dusty shops want fans that are easy to clean.
Worked example
Voltage: 24 V. Capacity: 220 Ah. Target 15–25% → 33–55 A.
A 25 A charger returns roughly 200 Ah in about 9 hours — too slow for a single overnight window if the pack is deeply discharged. Step up to a 40 A unit and you get there with margin.
Parts mentioned in this guide
Common questions
Can I use a 12 V charger on a 24 V pack?
No. Connecting a 12 volt charger across a 24 volt string either does nothing useful or charges only the tapped battery and imbalances the pack. Use a 24 volt charger, or a multi-bank unit that connects to each battery separately.
Do I need a bigger charger to charge faster?
Up to the 25% rule, yes. Beyond that you are trading battery life for minutes. If turnaround is the real constraint, an opportunity-charge routine (plugging in at breaks) usually beats an oversized charger.
What if my battery is a voltage nobody stocks?
We carry 8, 32, 42 and 72 volt units precisely because they are hard to find. If it is not listed, call the tech line — some models are build-to-order.