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Simple Question Regarding Isolating Car Batteries With Diodes

labant

Nov 12, 2011
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I'm looking to isolate a few car batteries I use for lighting in my garage (I live in an apartment, no power in the garage). So I currently use 3 different brand, different CCA batteries, all 12 volt, connected though a 400 watt power inverter. I usually connected them in parallel with jumper cables, but have come to realize that's not a good idea. I plan to build a connection rig to connect the batteries with much shorter, more secure cabling, and want to include battery isolation via a diode on one of the leads coming from each battery. I want to make this easy to upgrade to allow more batteries if I get more in the future. I feel like this should be an easy task, accomplished with a single diode on each battery. I can easily figure the wiring, it's just short battery grade wires connecting the terminals, no problem there, but the problems are: 1) which terminals should the diodes be connected to, positive or negative? 2) what diodes can I use?

Any help would be greatly appreciated, Thanks!
 

davelectronic

Dec 13, 2010
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Battery and diode isolation

Hi there. Batterys the same ratings in parallel or series should be ok, but for isolation diodes and heavy currents you will need stud mount units, as they make diodes in this catagoury to handle big power, you could use a square package full wave bridge rectifier, what ever you opt for there will be a volts drop across the diodes / rectifier, any where from 0.5 to 1.0 volt per diode / rectifier, but batterys fused in paralell should be ok so long as there identical.
Dave.
 

davenn

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Sep 5, 2009
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hi labant
welcome to the forums :)

you were wise to stop and ponder the different batteries
It is a bad idea to mix battery brands even if the stated voltage and current capabilities are "supposed" to be the same you can almost garantee that they wont.
They WILL have different discharge and charge rates.
Aim to replace 2 of them soon so you have all 3 the same. Do some info searching on the 3 brands and see which brand has the best characteristics and keep that one as part of the trio set.

BUT dont leave the swapping too long, else you will end up with one older battery mixed with 2 new ones. And again you would have variations in charging and discharging.
I have a ham radio friend who works for a large international alarm monitoring company here in Sydney, Oz. Their battery backup system uses 40 x 12V 120AH batteries. These must be replaced every 3 years regardless of if they need to be or not, at a cost of some $30,000. Many of the batteries still have lots of life in them and I generally get to score 3 of them every 3 years to keep my battery backup system primed up :)

cheers
Dave N
 

labant

Nov 12, 2011
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I'm not too concerned about the charge/discharge rates of the batteries since i charge them separately in the apartment. what am I looking for in peak reverse voltage, forward continuous current, max surge current, and power dissipation?
 

labant

Nov 12, 2011
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My thoughts are that I would need Power Dissipation of 400W or more since that's what my inverter is capable of, Max Surge Current of 800A+ to cover the absolute max potential CCA of the batteries, just in case, and Peak Reverse Voltage higher than the battery voltage. Would that be correct?
 

Resqueline

Jul 31, 2009
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Since you'll connect the batteries together only when they're to be drained I would not be too concerned about them having to be equal to avoid damage to them.
That's an issue only with permanently connected batteries, where one slightly bad battery will damage (drain) the other good ones.
I would use a triple gang switch if I could find one with a high enough current rating. 3 x 40A would be playing it safe, but 3 x 15A could do - assuming equal load sharing.
The other alternative is using diodes as suggested, but that'll cost at least half a Volt (using Schottky diodes). They can be found in PC power supplies btw.
The UPS power rating has nothing to do with the power dissipation in the diodes. It's the current the UPS draws (along with diode type) that determines diode dissipation.
Neither does the CCA of the batteries have anything to do with rating of diodes or wires. Use the UPS low-voltage fuse as a measure to dimensioning the battery setup.
Schottky diodes may only have a 20V reverse voltage rating but this is sufficient. They'll need heatsinking good for around 20-30W in total though.
 

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labant

Nov 12, 2011
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So, Since I plan on keeping the rig connected for a few days before needing to charge the batteries (I don't use the lighting every day or for more than a few hours at a time) and I don't want to deal with switches, diodes would work. ut the question now is, Would two of these (http://www.mouser.com/ProductDetail/STMicroelectronics/STPS40L15CT/?qs=M848ylJt4n0qmxCC5oJNKw==) work, screwed onto a largish chunk of aluminum, to connect 3-4 batteries in an isolated fashion? I'm guessing I connect, say, the positive poles of each battery to the outside leads, then connect the center leads together and to the inverter? Can I use somewhat smaller wire than typical 4awg battery cabling?
 

Resqueline

Jul 31, 2009
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Yes, those diodes should work, though you might consider using these for a little more "headroom". Yes, you can use half of each diode for each battery.
Observe that the cooling tabs will also be common positive so they can be used for wire connections, but must be insulated from any "negativity".
4AWG wiring is good for up to 100A and 37 feet. For 20A you can use up to 18 feet of 14AWG. A max of 29 feet of 8AWG is good for up to 50A.
 

foTONICS

Sep 30, 2011
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1) which terminals should the diodes be connected to, positive or negative?

Any help would be greatly appreciated, Thanks!

i dont think it matters which terminal you connect the diodes to as long as they are in the right direction
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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You can also use MOSFETs instead of diodes to do the diode-OR function. This is more complicated but much more efficient, and therefore more compact because much less heat is dissipated, due to the lower voltage loss.
The following page has an example schematic:
http://www.edn.com/article/518137-MOSFET_provides_high_power_at_low_loss.php
The reverse diode in each MOSFET does not defeat the diode-OR function, since the MOSFET is connected so that the current through it flows in the direction of that diode. The MOSFET simply parallels the diode and carries the current, giving a very low voltage drop due to the low ON resistance.
In your case, you would need pretty grunty MOSFETs, possibly several in parallel for each battery. Also I would use a smarter control system, so that it's not possible for any MOSFET to be operating in its linear region, where heat dissipation would increase. You might be able to use the above circuit with a bit of hysteresis added to the comparator, I'm not sure.
LT make an integrated device that does all of this, but it's only rated at 5 amps:
http://www.linear.com/product/LTC4358
They also make a very cool little device that will control an external MOSFET (or presumably several in parallel) and is probably ideal for your application:
http://www.linear.com/product/LTC4357
These are available in one-off quantity from DigiKey for about USD 4.00 but they're tiny SMT devices. Worth considering though IMO.
Good luck!
 
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