Charging batteries with a DC-DC converter

[mikedefieslife]

Was looking at the Victron Orion range of DC to DC converters, and noticed they mentioned battery charging as a potential use. This wasn't why I was looking, but it did get me thinking. Anyway;

As I understand it, the 12v DC-DC converter can output a voltage up to 25% higher than the input voltage. Theoretically then you could charge an AGM battery from a 12v FLA, however since it's not a charger, it doesn't have charging curves or profiles, nor a temp sensor, or cut in/out.

Is an AGM going to be happy being supplied with a constant/fixed voltage, and what is there to stop over-charging?

[john61ct]

So many variables. In theory could be done in certain circumstances, in a very limited way, with great inefficiencies involved.

But practically speaking, you want an active charge source putting out decent amps in order to charge a decent sized and well cycled battery.

And as you say, a multi-stage regulator, if you're not going to do the job yourself.

[CatNewBee]

there are b2b charger with IUoU charging programs in the RV world...

[CatNewBee]

they are called Battery Booster... In Germany very popular from Votronic or MT ... http://www.votronic.de/index.php/de/...e/lade-wandler

[boom23]

Search for 'dc to dc battery chargers'. Lots or products out there.

Your battery will be happier and last longer with a multi-stage battery charger.

[john61ct]

For high amp needs, Sterling is great.

BB series allows full adjustable charge profiles.

But such gear is bought much more frequently than it is actually needed.

[Locquatious]

When charging a battery, the charging source will match or be slightly above the battery voltage. If the source's current capacity exceeds the battery's acceptance rate (the rate at which the current to the battery does not lead to chemical storage of the energy), the voltage of both will rise and the excess current will be converted to heat in the battery. This will cause the battery to boil, which in AGMs and gel cell batteries leads to bubbles being entrained in the glass mat or the gel electrolyte. The bubbles may take time or never be re-adsorbed. In normal wet cell batteries, the bubbles escape. You just need to add water. With lithium batteries, excess voltage leads to a runaway reaction.

A DC-DC converter normally limits the voltage on the output and holds it steady. Current flow to the battery (if not limited by the converter) would match the battery's acceptance rate at that voltage. If the DC-DC converter voltage setting is right for the battery it will not damage it. So a DC-DC converter with a three-stage charging capability (an electronic chip costing just a few dollars) would work fine.

In the past it was argued that, for example, our Kiss wind generator didn't need regulation. Even though it might be charging the battery at 15V, it would be equalizing the battery and you only need to add water more often. Not sure if that was ever a correct argument and not sure if uncontrolled equalization is ever good, but with the modern batteries (AGM, Gel, Lithium) charging with an unregulated wind generator will damage the battery. This is one reason the Kiss Extractor™ has settings that control maximum battery charging voltage. When the battery voltage setting is reached, the wind generator stops for a period and then restarts when the voltage has dropped.

[john61ct]

Originally Posted by Locquatious

When charging a battery, the charging source will match or be slightly above the battery voltage.

No.

The charging source needs to hold a much higher voltage than the battery's resting V in order for any significant charge current to flow.

Originally Posted by Locquatious

If the source's current capacity exceeds the battery's acceptance rate (the rate at which the current to the battery does not lead to chemical storage of the energy), the voltage of both will rise

No.

Any decent charge source will regulate voltage so as to not exceed the user selected setpoint. If voltage is allowed to rise for any reason, the charger is defective.

It is common, normal and healthy for a charge source to "offer" an available charge current much higher than the battery will accept at a given SoC.

Even a 400A source on a 30A lead battery will do no harm.

With chemistries capable of accepting very high amps, a charger with a temp sensor wire on the bank will prevent overheating.

Obviously robust connections, thick wires, proper fusing etc are important for safety.

[Locquatious]

Quote:
Originally Posted by Locquatious View Post
When charging a battery, the charging source will match or be slightly above the battery voltage.
"No.

The charging source needs to hold a much higher voltage than the battery's resting V in order for any significant charge current to flow. "

I think you misunderstand. Look at it this way. What is the wire resistance? Say 1 milliohm. And say that we are charging source-to-battery at 100 amps. E=IR says the voltage drop over the wire is 0.1V. That means that the source is slightly above the battery voltage by 0.1V.

Quote:
Originally Posted by Locquatious View Post
"If the source's current capacity exceeds the battery's acceptance rate (the rate at which the current to the battery does not lead to chemical storage of the energy), the voltage of both will rise."

I think you misunderstand. I am explaining why the voltages would rise and not discussing presence or lack of regulation with this statement. If the battery cannot absorb the current, the voltages have to go up.

But as you say, if the source voltage is limited, the voltage on the source and battery will remain at that limit. The current will eventually drop as well.

Say the voltage is 14.4. If you are putting 400 amps into a battery that can only accept 100 amps (conversion to chemical energy) you are just creating 14.4 X 300A = 4320 Watts of heat.

Think of it like a muscle car floored at a stoplight. Accelerating the car to gain speed or absorb higher potential energy can only happen so fast. So the tires spin and smoke and melt.

[roland stockham]

The one place this would be a good option is for a 'standby' battery. If you need a battery to back up a system but which is normally not discharged then a constant voltage applied at the float level is ideal. As an example this would apply to the back up battery in a burglar alarm system or possible a backup for the nav system that provided a stabilized voltage and emergency supply should the main bank fail. Any battery that is regularly cycled needs a multi step regulated charger.
Batter to battery regulated chargers that boost the standard voltage provided by the on-board voltage regulator in the alternator to a level where it can run 3 step charging are available but are more expensive than fitting an external regulator. The are primarily aimed at RV & emergency vehicles with computer controlled engines. In these situations you cannot use an external regulator as it would cause the computers to think there was a fault in the alternator or possibly fry the computer. Does not apply to any marine diesel I know of.

[CatNewBee]

The DC-DC Charger transforms low voltage / high amp to higher voltage lower amp to push the current to the second battery. Its a step-up converter followed by a 3-stage charge controller.

[Adrian Chen]

Originally Posted by mikedefieslife

Was looking at the Victron Orion range of DC to DC converters, and noticed they mentioned battery charging as a potential use. This wasn't why I was looking, but it did get me thinking. Anyway;

As I understand it, the 12v DC-DC converter can output a voltage up to 25% higher than the input voltage. Theoretically then you could charge an AGM battery from a 12v FLA, however since it's not a charger, it doesn't have charging curves or profiles, nor a temp sensor, or cut in/out.

Is an AGM going to be happy being supplied with a constant/fixed voltage, and what is there to stop over-charging?

We use Mastervolt Magic DCDC converters that have built in 3 stage charging to charge our AGM thruster batteries. So far after four years we have had no problems. We have bow and stern thrusters, so two separate installations.

[john61ct]

Do they charge from source bank to the target ones when there is no primary charge source active?

Is the current rate in that scenario significant?

What are the distances between banks?

[CatNewBee]

It depends on the configuration. Usually they start at a given Voltage (like a voltage based battery combiner), this ensures you will not drain the start battery. They only run when the alternator delivers power. Some have a separate D+ input from the regulator, so they kick in on that signal.

Many have DIL switches to configure the load and the cut-off + the battery type.

[Adrian Chen]

Originally Posted by john61ct

Do they charge from source bank to the target ones when there is no primary charge source active?

Is the current rate in that scenario significant?

What are the distances between banks?

They will charge from the source bank when the voltage on that bank is greater than a user preset trigger. I have them set on 13v so they only come on when a charge source is active, however this could be lowered. If I remember correctly (and I'm old and fallible) you can also preset a cut-out voltage to protect the source.

The current rate is a maximum of 20amp, (with my MAGIC however the battery will only draw what's appropriate for the voltage at float/absorbtion up to the maximum (the MAC plus version of these go up to 50amp. The main difference between MAC and MAGIC is the inclusion of galvanic isolation in the maGIc) depending on the stage.

The banks are about 20ft from the source, but cable runs are a longer of course. Domestics are in the mid point of a 47ft boat.

We have them set to operate when a charge source is available because I normally want to keep the domestic bank as high as possible, the thrusters are rarely used much in any case. One downside of this arrangement is that the thruster battery/wiring must all be good, because there is no help from the alternator output as there might be in other installs