Charging small 12v battery from 24v bank

[Dave100456]

Advice please:
I have a small; 12v 7ah battery that I wish to charge from a large; 24v 240ah house battery bank when sailing.
Has anyone sourced a suitable charger, I'd prefer not to go down the inverter then 230v charger route?
Can I use a solar controller, connecting the 24v bank where the pv panel would normally be?
Thanks
Dave

 

[sarabande]

24v to 12v transformer. AS used in lorries.

http://www.amazon.co.uk/Truck-Reducer-watt-Power-Transformer/dp/B004GVA3DI


you may need to search other sources if the proposed charge rate is above 5 amps though.

 

[Dave100456]

24v to 12v transformer. AS used in lorries.

http://www.amazon.co.uk/Truck-Reducer-watt-Power-Transformer/dp/B004GVA3DI


you may need to search other sources if the proposed charge rate is above 5 amps though.

Thanks
I think this only 'transforms' from 24v to 12v. I'd like a version that has a 3 stage charging contol function too. This must be a common need but I can't locate a ready made device.

 

[sarabande]

what about a solar panel regulator ?

e.g. Sun and Aire

http://www.aireandsun.co.uk/index.php?maincat=Solar Charge Controllers

Some of the clever ones moderate the input from panels to batteries - which is basically what the 24 to12v unit is.

 

[stuhaynes]

24v to 12v transformer. AS used in lorries.

http://www.amazon.co.uk/Truck-Reducer-watt-Power-Transformer/dp/B004GVA3DI


you may need to search other sources if the proposed charge rate is above 5 amps though.

I had a 24v to 12v voltage dropper to run a 12v water pump during the process of conversion from 12 to 24 volts. The damn thing got so hot during the (relatively short use of the pump) process that it could easily be mistaken as a radiator, a very hot one at that. Downright scary, hot enough to burn! Unless the technology has been much improved in the last 5 years I'd be tempted simply to tap straight off of one of the 12 volt batteries that make up your 24 volt system.

 

[Dave100456]

I had a 24v to 12v voltage dropper to run a 12v water pump during the process of conversion from 12 to 24 volts. The damn thing got so hot during the (relatively short use of the pump) process that it could easily be mistaken as a radiator, a very hot one at that. Downright scary, hot enough to burn! Unless the technology has been much improved in the last 5 years I'd be tempted simply to tap straight off of one of the 12 volt batteries that make up your 24 volt system.

Thank you Sarabande and Stu. I had thought of the solar controller as mentioned in my original post. However, has anyone tried it?
With a PV panel the wattage is constrained by the size of panel and if I replace the PV with a 24v battery bank the available wattage is very much larger and could cause problems.
I have seen the Sterling battery to battery chargers but I am looking for a low cost charging solution for a portable device with a 7ah battery that I can plug in when its not in use.

 

[William_H]

Battery charging

The 7AH SLA battery will be quite sensitive to overcharge so be careful. If you need rapid recharge then you need a 3 stage charger. The problem with solar controllers is that they reduce the voltage by resistance which gets hot. The philosophy is that the power from the panel is there to be wasted. From a 24v battery system you could end up taking a lot of 24v and producing a lot of heat. What you really want is a switch mode type regulator charge controller which reduces the voltage by taking bites of full power at an on off ratio which produces the output needed. You may find a solar regulator charger which does this.

However if you only want to ensure the 7AH battery is always charged but can be charged by a trickle. ie if there is generally a light load on the battery you could use just a series resistor to feed a trickle into the battery. The resistor calculations are that you need to drop 14v at a current that you might choose. ie 5 milliamps for continuous charging requires a resistor of R=E/I 14 divided by .005 (A) is about 3000 ohms which will radiate heat of 14v X .005 or 70 milliwatts. For double the current 10ma the resistor is 1500 ohms and the power is 140mw.
The current needs to be chosen low enough to not harm the battery for 24/7 charging but high enough that any battery use will be recharged in a suitable time. 5ma will take 700 hrs or more to recharge a 50% discharged battery. I would not float a 7AH at more than 20ma for long life of the battery but maybe maore is OK. If there is a regular discharge of course then more charge is indicated.

For loads on the battery of more current. You can use a purpose built regulator chip. 7812 is the generic part number. This will give max 1 amp but will need heat sink to cool it. The voltage can be increased by fitting a silicon diode in the earth connection so bringing regulated voltage to 12.7 or 2 diodes to 13.4 v. Or use a LM317 with variable resistor to adjust voltage to what you want.
This arrangement will float the battery at 13.4 volts and provide any load up to about 1 amp.

For larger loads the 24 to 12v converter built for the purpose would be best. Useful on a 24v boat for 12v loads anyway. These usually give 13.75 volts out which will float charge the battery and provide 5 amps or more of load.
Note that all of my suggestions do not involved staged battery charging. This is OK but means that recharge of a flat battery will take longer.Considering that for about 80 years we have run cars successfully with a single regulated charge voltage this may not be a bad thing. It is just that a staged charger can do it quicker. PM me if you have questions olewill

 

[Dave100456]

William, Thank you for the full and detailed options. I have pm'd you

 

[Rigger Mortice]

http://www.intellitecmv.com/products.asp?gid=19&rid=30&d=110