Motorhome and Caravan Technical Articles

 Every vehicle has an alternator, we know that it is responsible for keeping the starting battery from going flat.  But can we use the alternator to charger our motorhome house batteries too?  What is the best way to do this?

 

Before we look at the options, let’s look at the issues:

1. Crank batteries differ in their construction and charging requirements when compared to the deep cycle batteries that we use in our house systems.
2. Flattening the crank batteries must be avoided at all costs. Trying to start a motorhome with a flat battery is no fun.
3. We must never (accidently) try to start the engine using our house batteries.

 

Here are five ways of using your vehicle engine and alternator to charge your motor-home house batteries. I’ll start at the cheapest option and work my way up to the most expensive (and perhaps best) option.

Option 1 – Switch on the dash

With this option you only need a very heavy duty switch (perhaps 100amp rating or better) that simply connects the positive terminal of the house batteries to the positive terminal of the crank batteries. When the switch is on, current from the alternator will charge both the house batteries and the crank batteries. When the switch is off the two battery banks are not connected (thus preventing you from flattening the crank batteries at the same time as the house batteries).

 Advantages

• Simple to install
• Cheap (approx $100)

Disadvantages

• Manual – you have to remember to switch the connection on and off
• Forgetting to turn the switch off could result in both sets of batteries being flat, leaving you stranded
• The house batteries can only be boosted – not fully charged.
• Does not prioritise charging of the batteries (ie can not charge the crank battery first – then switch to house battery).

 

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Option 2 – Ignition controlled relay

This is really just a refinement of option 1. Here the manual switch is replaced by a relay (suitably rated) and this relay is activated (ie powered) when the ignition switch is turned on. This removes the manual switching disadvantages of option 1. Note – if you are considering using this option, be very sure that you use an accessory feed from the key switch. Make sure that this deactivates the relay BEFORE the starter motor engages (most accessory feeds do this). This is important to avoid drawing power from the house batteries while starting the vehicle.    

 Advantages

• Fairly simple to install
• Cheap (approx $150)

Disadvantages

• The house batteries can only be boosted – not fully charged.
• Does not prioritise charging of the batteries (ie can not charge the crank battery first – then switch to house battery).

 

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Option 3 – Voltage controlled relay

Another refinement of option 1 – a bit smarter. A special relay is fitted to interconnect the two battery systems. This relay is able to monitor the voltage of the crank battery and join the two battery banks together ONLY once a sufficiently high voltage has been reached. This effectively prioritises the charging – making sure the crank battery is charged before joining the two batteries together.   Unfortunately, if the house batteries are very flat, some power will immediately flow out of the crank batteries and into the house batteries.    

When the engine is turned off and the alternator stops producing power, the voltage of the crank battery will quickly fall and the relay will seprate the battery systems.

Advantages

• Fairly simple to install
• Cheap (approx $200)
• Does prioritise charging of the batteries (ie charges the crank battery first – then switches to the house battery).

Disadvantages

• The house batteries can only be boosted – not fully charged.
• Current can flow out of the crank batteries and into the house batteries (and visa-versa).

 

 

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Option 4 – Diode isolation

To avoid the issue where one battery bank can feed the other (flatter) battery bank, it is possible to fit diode isolation. This is simply a box with 3 terminals on it. A diode allows current to flow in just one direction. You can then be sure that power can flow into the battery (via the diode isolator) but not back out and into the other battery.

The big disadvantage of this system is that diode isolators introduce a 0.6 volt drop in the system. Now this does not sound like enough to worry about, but it is quite significant. 0.6 volts could mean the difference between charging your batteries to 65% or 85%.  My advice is to avoid diode based isolators unless you know what you are doing and are able to overcome the 0.6 volt issues (and this can be done – write to me if you want to know how).    

 Advantages

• Fairly simple to install
• Cheap (approx $100 – $200)
• Stops discharge from one battery into another

Disadvantages

• The house batteries can only be boosted – not fully charged.
• Both sets of batteries will receive less charge (due to the voltage drop across the diodes).

 

 

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Option 5 – Two alternators

With this method you simply fit a second alternator to the engine. The two battery systems remain completely isolated from one another. The original alternator charges the crank batteries as it always did. The additional alternator is responsible for charging the house batteries. This is actually my preferred option. Electrically speaking it is simple (mechanically it can be difficult to find room and fit the second alternator). This option avoids most of the pitfalls of the previous methods by keeping the two batteries isolated from each other. It is also possible to replace the regulator on the house alternator with one that more closely matches the preferred charging characteristics of house batteries.        

 Advantages

• Electrically simple
• Can be setup to more fully charge the house batteries
• Avoids discharge from one battery into another
• No switching or manual intervention required

Disadvantages

• Mechanically tricky – it is not easy to fit a second alternator
• More expensive (say $300 – $600)

 

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 Conclusion

There are other ways of using smart regulators or even inverters powering 3 stage chargers to your house batteries. Presented here are just 5 of the simpler and cheaper options.

You should also be aware that charging from your alternator is not free (the bloke that first said “there are no free lunches” was a physicist for sure). When you use your vehicle engine to charge your batteries your vehicle will have reduced power and/or consume more fuel. However, this increase in fuel consumption compares favourably when compared to the fuel consumed by the typical small generator.  So the lunch is not free – but it is quite cheap.

Tags: battery, Caravan, Electronics, Energy, Motorhome, Power, Technology

 

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This entry was posted on Monday, September 28th, 2009 at 4:56 pm and is filed under Caravan, Motorhome, Power. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.