Thread: DC/DC chargers, How do they work and do you really need one.

Originally Posted by Barraman

"1. A VSR can charge 'house' batteries more quickly than a DC-DC converter."

My experience, with a VSR and then a DC-DC charger in my boat, doesn't support that statement!

Agreed, the word is "can" charge more quickly, not "will" charge more quickly, and when it does, it will only be the first percentage that will charge more quickly. For a FULL charge, the DC-DC is likely to be quicker.

Originally Posted by drivesafe

Hi Deano and not real up on battery technology mate.

Spent a lifetime working with batterys of many types Tim and in many applications. Lead acid mainly, with a bit of NIFE thrown in for good measure . More recently with LiFePO4, I have no experience with lead crystal but technologys advance and life is a continual pursuit of knowlege.

Originally Posted by drivesafe

.......................... if you have a back of batteries of different chemistry, they WILL NOT continually discharge from one battery type to another, regardless of type, age, or side.

if the batteries have been charged together, via an alternator, you can have batteries with a slightly higher SETTLED voltage when you first turn the motor off.

At this time, the the batteries with a higher voltage will slowly discharge back into the batteries with the lower voltage.

Then once all batteries are at the same voltage level, the discharging of ALL BATTERIES stops.

No problem here Tim, with you all the way , as you say the the batteries with a higher voltage will slowly discharge back into the batteries with the lower voltage. Differences in 'settled' voltage between different age/type of battery will mean the battery bank will 'equalise' to the battery with the lowest voltage. This is a waste of power / battery capacity and can be avoided by using batterys with the same characteristics. ie. similar condition/technology.

Perhaps the worst example of this in an off road scenario is when a paralled battery 'drops a cell' ie. one of the batterys cells physically fails (usually due to vibration/corrugations) and becomes shorted internally. Whilst the engine is running the majority of the alternator output power is dissipated in the faulty battery which rapidly becomes very hot and in a flooded cell battery can cause the electrolyte to boil and 'geyser' out of the top of the cells. A sealed battery can blow up like a football and even physically break open. A very dangerous situation.

This happened to me on the Kalumburu Road several years ago with a solenoid coupled (same as a VSR) main/aux battery setup. Unfortunately the 'equalising' currents involved between the batterys was so great the solenoid contacts welded together so the batterys were electrically coupled even with the engine stopped and power removed from the solenoid coil. A swift clout with a very large rock to the solenoid coil shocked it into release and many gallons of water was used to wash away the acid and reduce the failed batterys temperature. Using an electronic battery coupling device instead of a VSR may 'solve' this issue. I configure paralled batterys now so that major failure of one battery can't 'take out' the rest.

Whilst this is an extreme example of having a very mis-matched battery bank I'm not a fan of paralleling old and new batterys together for the reasons above.

Originally Posted by drivesafe

.......................... This is exactly how my isolators work. Allowing a higher charged auxiliary battery to slowly back-discharge into a lower charged cranking battery and thus, keeping the cranking battery in a higher state of charge than it would be in normal use.

Sounds like a good idea to me Tim as long as you can ensure that the aux. battery is the higher charged one

Originally Posted by drivesafe

When a lithium battery is connected to any form of lead acid battery, the lithium battery will continually discharge back into the lead acid battery until the lithium battery is discharged down to 12.7v, at which point, the lithium battery will be nearly flat.

Which is another good reason to use a suitable DC-DC converter to charge a lithium house battery as it provides back feed isolation, well my Votronic one does .

Deano