Charge batteries when loads are intermittent or surge at start-up, and supply AC directly when the loads are steady and fit within the plant’s constant output. A 350 W plant makes 8.4 kWh a day but cannot start a 250 W compressor that surges to 750-1,500 W, whereas a battery bank and inverter can, and a bank that absorbs the plant’s full 24 A at the gentle C/10 rate needs about 243 Ah.
Part of Micro-Hydro Turbine
A hydro generator can feed a battery bank through a charge controller, or feed alternating-current loads directly with an electronic load controller holding its speed steady.
Two Ways To Use A Steady Stream Of Watts
| Battery charging | Direct AC | |
|---|---|---|
| Chain | generator, rectifier, shunt charge controller, batteries, inverter | generator, load controller with dump load, loads |
| Peak power available | what the inverter and battery can supply | only what the turbine makes |
| Energy kept | stored for later | wasted as heat in the dump load, unless the heat is useful |
| Extra parts | batteries, controller, inverter | load controller, dump load |
| Main risk | battery failure and its gases | frequency and voltage drift if the controller fails |
Worked Example: One 350 W Plant
Energy. 350 W × 24 h = 8,400 Wh = 8.4 kWh a day. A stream flows all day, so this is not a peak figure (Power Calculation).
Starting a motor. The Load Control page says a motor draws 3 to 6 times its running current on starting. A 250 W compressor therefore asks for 750 to 1,500 W for a moment. On direct AC the plant makes 350 W, so the generator is dragged down and voltage and frequency sag. A battery and inverter supply the surge from stored charge.
Charging current. At 14.4 V (the top of the charge range on the Charging Systems page) 350 W is 350 ÷ 14.4 = 24.3 A. That page charges at C/10, a tenth of the battery’s amp-hour capacity per hour, so the bank should be 24.3 × 10 = 243 Ah at 12 V. A smaller bank is charged faster than that rate; the shunt controller then sends the excess to the dump load.
A bigger plant changes the answer. A 2,800 W plant would run a 750 W-surge motor directly. The case for batteries is strongest on the small plants, where a single appliance’s surge is large compared with the plant’s output.
Battery Route: Use A Shunt Controller
The charge controller must be the shunt type, which diverts surplus to a dump load rather than disconnecting the generator. A series controller opens the circuit when the battery is full, which unloads the turbine, and the turbine speeds up exactly as it does when a breaker trips. The reasoning is on the Charge Controller page, written for wind and equally true for hydro. For the battery itself see Lead-Acid Construction. Installation advises a bank for 2 to 3 days of autonomy, the energy needed if the turbine is shut down for maintenance (Installation).
Direct AC Route: Hold The Speed With A Dump Load
With no batteries the generator frequency is set by the turbine speed. The electronic load controller keeps the total load constant by feeding the dump load whatever the real loads do not take, and the dump load is sized at 100 to 120% of the turbine output (Load Control). The loads see a stable frequency but cannot draw more than the plant makes. A water-heater element in a tank turns the surplus into hot water, so little is wasted. Voltage regulation is on Voltage Regulation.
Pick Batteries For Surging Loads And Direct AC For Steady Ones
- Lights, phones, a radio, a fridge: batteries, since the fridge compressor surges and the demand is uneven.
- Water heating and a workshop with large steady machines: direct AC, using the surplus as heat.
- Both: run the lights from the battery bank and the dump load as a water heater; this is a workable arrangement for a small village.
Hydro needs a battery for peaks; wind needs one for calm
Hydro makes the same power day and night, so its battery only covers peaks and shutdowns. A wind turbine can make nothing for days, so its bank must hold several days of use (Battery Bank Sizing).
Common Mistakes
- A series charge controller on a turbine — it removes the load and the turbine overspeeds.
- A bank charged too fast — the plant’s current exceeds the C/10 rate that the Charging Systems page recommends.
- Direct AC with no dump load — the first lost load speeds the generator up.
Safety
Battery banks and AC supply
A lead-acid bank can deliver hundreds of amps into a short circuit, so fit a fuse at the battery terminal. Charging gives off hydrogen, which explodes; keep banks ventilated and free of sparks and open flames. Keep inverter and generator wiring at 120 V or 230 V away from water (Electrical Safety).
What’s Next
- Learn next: Maintenance & Troubleshooting to keep the whole chain running
- Combine with: Inverter Basics for sizing the AC side