ideas for a battery balancer in a landcruiser?

[stumitch]

hi all! anyone suggest a battery balancer circuit for a 24volt toyota landcruiser? the alternator/battery circuit is 24 volts but i need to draw 12volts for lights, stereo...etc...i can tap 12volts off one battery but when one battery drains more than the other a charge problems happens...batteries go boom...

 

[JTECH]

12v From 24v

Hi!
For light (low current loads) you can use a DC to DC converter, but for large loads, a very simple solution (also used on big cooler trucks) is to stack about 18 to 20 high current diodes in series! (Dropping diodes). A cheaper way is to use about 8 or 10 insulated bridge rectifiers in such a fashion that all the internal diode pairs are in series between +24V and the load.(No connections are made to the ac terminals of the bridges).
Hope it helps!

 

[kchriste]

For light (low current loads) you can use a DC to DC converter, but for large loads, a very simple solution

I would say the opposite. Diode trick for the light loads. Too much heat otherwise.

Take a look at ICT power supplies:
**broken link removed**

 

[JTECH]

12v From 24v

Heat is a minor problem under the hood of a vehicle vs. cost and reliability. I have used this method on a number of ocasions with success. The bridge rectifier method is easy because they are insulated and easy to mount on a piece of aluminium. DC to DC converters (switched mode) are the obvious high-tech solution, but at a price! If that's O.K. with you, you'll have to find one with a safe current rating for your application(s).

 

[stumitch]

dc converters...

thanks for the replies...the load isn't huge...lights, stereo (not a huge bass-thumper...) i'd like to try the bridge rectifier idea. simple is good although what about making a dc-dc converter? too tough? do-able?

 

[Hero999]

Bridge rectifier?

That'll only give a 2V drop at the most and that's a full load.

Still it's only a good idea to use diodes fo light loads, (I'd say below 100mA) and use a zenner rather than loads of diodes. For larger loads (100mA to 5A) use a linear regulator.

Your lights and stereo probably draw considerable current.

Can't you change the light bulbs for 24V types, they're not expensive?

I would also recommend buying a 24V stero, failing that then DC-DC converters are quite easy to build or you can buy a module.

What fuse do you need for your lights and stereo, use the current rating as a rough guide.

 

[kchriste]

Heat is a minor problem under the hood of a vehicle vs. cost and reliability.

Good point. He could use the vehicle body as part of the heatsink. Not very efficient though. Cheap, yes.
For the lights he could wire 2 12V fixtures in series as long as the casings aren't the "ground side" of the lamps. eg: 2 fog lamps etc....

 

[Sceadwian]

It almost sounds like it'd be easier to re-wire the rest of the car for 12 volt operation... Or mount a second alternator for an independant 12 volt system.

 

[JTECH]

12v From 24v

Putting in another (12V) alternator is another option, one that I personally would not do due to the extra mechanical work involved and also because this will put an extra burden on the engine, causing it to be even heavier on "juice"! In this instance, it will be cheaper to install a DC to DC converter as was suggested by one of the members earlier. It will also be a lot easier to install.
Have you calculated your total load requirement (for the 12V devices and equipment)? This is the right place to start before any dropping diodes, DC to DC converter or even 12V alternator is purchased.

 

[JTECH]

HERO999 is correct in mentioning that you could change a lot of things to 24V. That way, you'll only need a small converter or diodes or bridge rectifiers to power the light loads.
As far as the volt drop of 2V per bridge is concerned, I have found that by using 35A bridge rectifier blocks (in aluminium casting with hole in the middle for mounting), that the actual volt drop across each bridge was closer to 1,6 volts and that was at a current of about 6 amps. With a vehicle voltage (engine running) of 28.4V, you'll need 10 of these bridges to give you roughly 12V out. Considering that your 12V equipment can in any case withstand 14.2V, you can do away with one of the bridges and you'll have 9 instead of 10. Granted, this may differ using different manufacturers' rectifiers and different values of load, but then that's not the end of the world, because automotive equipment can tolerate a wide variation in voltage. I would start with 10 bridges and put the load(s) on, measure the voltage at the load(s) and gradually reduce the number of bridges until I am satisfied with the results.

Note: This method is still widely in used in substation tripping supply units (battery chargers) to protect the critical load during a boost cycle when the voltage to the battery would exceed the voltage required by the load.

Good luck!

 

[Hero999]

2V was a general figure based on typical 1N4001 diodes, more expensive diodes will have a much lower drop and schottky diodes will be even lower.

But why use 10 diode bridges which will give you poor regulation when you can just use one zener or even better a linear regulator which will give you excellent regulation?

Using 10 bridge rectifiers seems a waste of time and money especially when you'll only be using half the diodes! The other halves of the bridges will never conduct because it's DC current not AC.

On a good heatsink the LM338 in a TO-3 package should give you 5A@14V, and the LM317 can give 1.5A, you can also turn it into a fairly crude switching regulator, see the datasheet.

How much do those bridge rectifiers cost?

I bet it's cheaper to build the switching regulator on the LM317's datasheet than it is to buy all those bridge rectifiers, it will also be more efficient and give better regulation too.

 

[Nigel Goodwin]

I'm still a little confused about the Land Cruiser?, it's 24V (fair enough), and the stereo is 12V (again, fair enough), but what are the 'lights' you refer to?.

BTW, 24V to 12V converters for stereo's are a common component for lorries and trucks!, they used to be linear ones, but there may well be switch-mode ones available now?.

But the 'lights' are a different matter, presumably it's not the vehicle lights?, as a 24V vehicle would have 24V lights.

 

[Oznog]

The diodes are bordering on a ludicrous solution.
You need a DC/DC converter. These aren't too difficult, it's a controller, a transistor, an inductor, high speed diode, and output cap. It needs to be sized according to your load.

Output regulation with one of these is a bit tricky. At its simplest, you could just use a small 12v sealed lead acid battery, like maybe 7 amp-hrs, to better regulate the power and better yet supply voltage when the converter is off to keep the presets in the stereo alive, keep a clock running, etc.

 

[chiba]

Umm, how much current are you drawing? If your lights are spotlights or driving lights then you'd be sucking a lot of juice. It may be better to have another 12v battery and step down your 24v to charge the extra battery?
Then again you could put some pass transistors on a 12v reg. **broken link removed**
I wouldn't do anything till i knew the load required.

 

[JTECH]

Nigel, Oznog and Chiba are all very correct and I agree 100% if good regulation is what you want and if you don't mind the price.

Bridge rectifiers (35A) costs around US$1.14 or 57p (Chinese imports). They don't get all that hot when mounted properly and it does not normally feed a load 24/7 in a vehicle.

When you are in the bush with your 4 x 4, you do not always have access to things like LM338 regulators and DC to DC converter spares, but you may find bridge rectifiers almost everywhere in the 3 rd world (where 4 x4 vehicles roam). However, when left with a choice I would also prefer the more high tech solution(s) as was(were) suggested, with the exception of the zener diode idea. Assuming that your equipment takes 6A, the series resistor alone will have to drop 16.4V at that current (44.83 watts!). To stay out of the orange part of the visible spectrum, a 100W resistor is the practical value! Then, at zero load current, the poor zener will have to be tough and may not be commercially available!

Finally, ALL FOUR DIODES in each bridge are used when bridges are wired in SERIES, omitting the AC connections to the bridges; i.e. from +24V side of supply, ENTER at bridge#1 (-), current splits and flows in 2 parallel paths through the bridge and combines again at the (+) terminal of that bridge (4 diodes used) (EXIT). This then feeds bridge#2 (-) and so on....Make a schematic diagram of bridge rect. to verify for yourself.

Sorry for giving you a tough choice!
Good luck!

 

[JTECH]

**broken link removed**

 

[Hero999]

Sorry but it's still a silly idea, it doesn't make sense from either a technical or econnomic perspective.

An LM338 and LM317 are very easy to get hold of, both are cheaper than one of those rectifiers and if it isn't powerful enough by itself then adding a 2N3055 to boost the current won't cost much.

You could probably could build an LM317 switching regulator for less than two of those rectifers.

 

[kchriste]

When you are in the bush with your 4 x 4, you do not always have access to things like LM338 regulators and DC to DC converter spares, but you may find bridge rectifiers almost everywhere

I would like to know where in the bush that the bridge rectifiers grow. I can't seem to find any in the forests here in Canada...