Tasman Energy
Home made direct charging plant

Battery Chargers Versus Direct Charging

The downfall of most solar systems is that when the sun is not shining or power use is high some sort of back up charging method is required. Most folk use a battery charger attached to a petrol or diesel generator. Well battery chargers suck. There are not many good ones on the market, most fail prematurely and I have yet to see one that will charge at a high enough voltage to get a deep cycle battery really freely gassing. Most quit providing current at around 2.3 - 2.4 volts per cell.

In addition to this, the rating of most battery chargers is optimistic in the extreme. What has often happened is that an engineer has calculated what the battery charger "could" put out under perfect conditions with an unlimited power supply connected and the charger connected to a battery with zero internal resistance. When you get home and connect one of these devices to an used battery bank with a small petrol generator you are then disappointed to see that the expected output is way below what you had been led to believe. The spec of a battery charger may look like this: 80 amps @ 24 volts

Let's do a couple of sums on this

80 amps x 24 volts = 1920 watts, awesome you think ... but when you connect it to a 24 volt battery bank, even a flat one, the terminal voltage of the battery sits around 26.5. 1920 รท 26.5 = 72 amps. suddenly the "80 amp" charger you paid heaps for has become a 72 amp charger. While in reality the energy is still the same, what you really wanted 80 amps all the way up to 30 odd volts. Perhaps you should have looked at a 2400 watt battery charger (100 amp rating) but the the petrol generator would be working harder and may not have the grunt ...

Then the next problem arises, you get home and read the fine print in the spec sheet and discover that the battery charger you have bought is limited to around 28.5 volts. If you refer to the operating manual for your battery bank you discover that the recommended charge voltage is actually 31.2 volts (2.6 volts per cell, source Exide Energystore manual). This charger isn't ever going to get these batteries full of charge! Sure it functions at a backup but it is not really that good for your bucks.

More fine print

You get this 80 amp charger home and decided to give it a go. Because you have been low on power for a few days you have neglected the washing. Beauty, you think, let's get the battery charger cranking then load up the trusty top loader with dirty cloths.

All goes well until the washing machine hits the spin cycle. Suddenly the battery charger fails. What happened? You truck the charger back to the supplier who ships it off to the agent or manufacturer. they fix it but refuse warranty because the device has suffered a voltage spike. You get a bill. (Don't laugh, I have seen this happen a lot).

What really happened is this; somewhere in the fine print may have been a spec stating the maximum load on the battery terminals at the time of charging. You neglected to notice and no one told you. Typically the maximum battery load while charging with a conventional battery charger is around 10% of its charge rate. There was no voltage spike from the generator but there was a huge voltage spike from the inverter when the load suddenly changed during the washing machine spin cycle.

Direct Charging

A direct charging machine will have none of the above limits or problems!

Direct charging is a term coined for battery charging machines that produce battery charging current instead of the more common AC "mains" voltage most generator produce. Typically these machines are engines connected to heavy duty car or truck alternators. There are a few on the market "ready built" but if you want a good low cost battery charger of high quality then you should consider building your own direct charging machine.

As well as heavy duty car and truck alternators there are a number of easily obtainable permanent magnet generators available that provided efficiencies batter than what is available with alternators.

The advantages of direct charging batteries

The advantages of a direct charging plant over the more conventional battery charger and generator could be listed as follows:

  • Efficient high rate charging from a relatively small engine
  • Ultra reliable battery charging
  • High charging voltage is easily obtainable
  • Current remains high all the way through to end point charge voltage
  • End point charge voltage is often adjustable
  • Small compact and portable units are easy to make
  • Inverters can be used without limit while charging

Here are a couple of examples

A family near here has a 24 volt 1330 amp/hour battery bank and a solar array of around 1 kW. In summer the solar array provides them with sufficient electricity for their needs most of the time. In winter it is a different story, this is Tasmania after all.

Referring back to my battery charging page you will see that a 1200 amp/hour battery bank requires a battery charger of at least 10% C10 capacity. The C10 capacity of this battery bank (Exide Energystore 4RP1330) is 800 amp/hours so the minimum battery charger that is required is a unit of around 80 amps. While battery charging is happening the owner would like to wash cloths or use the dishwasher or something similar while charging. This guy's name is Brian by the way.

What Brian did eventually was get a small diesel engine from a car wrecker. It was from one of those chronically underpowered vans you sometimes get stuck behind on a steep hill.

To this engine he connected 2 x 24 volt, 55 amp, truck alternators and regulated them with a single custom built variable voltage regulator. The result is a 110 amp charger that works perfectly as a battery charger and a backup generator (in conjunction with his inverter).

At another house not that far away from Brian's there is a 5.5 HP Honda stationary engine connected to a single 55 amp truck alternator providing backup power to another 24 volt battery bank. This one has been in place working trouble free for over 10 years. It punches out 50 amps at over 30 volts when required.

At my house I have a 48 volt battery bank. My direct charging plant is an electric start Kubota stationary diesel engine rated at 6 HP connected to a permanent magnet generator that started life as a DC motor. This unit puts out 70 amps at 60 volts and cost around $2500.00 to build. Try pricing a generator and battery charger that will do this!

Details of all this and more are in a new book: Build Your Own Direct Charging Plant

"Build Your Own Direct Charging Plant was first printed in 1992. It got a bit out of date and only included alternator charging. Things have changed, permanent magnet motors and alternators have come onto the scene and things have advanced somewhat so I have re-written this book from the beginning. It is all new and up to date for 2014

vintage Lister used as a direct charger

The above Lister engine is a 3.5 HP water cooled diesel engine made around 1950. It is connected to a 2 kW permanent magnet electric motor and outputs a charge current of 40 amps at 48 volts.

Tasman Energy sell a book titled "Build Your Own Direct Charging Plant" which details this and other direct charging plants.

More details on this book