How does a battery to battery charger work?

[BozSail]

How does this differ from a regular battery charger, and what is its use on a boat, if any?

[john61ct]

AKA DC to DC charger, but many different types, many based off alternator charging only.

Now some also incorporate a basic solar controller.

The key element is DC input as opposed to shore power.

Like a VSR / ACR but one way only, and with full three stage intelligent charging on the output, voltage etc setpoints independent of the source, as opposed to just passing the same voltage through.

[john61ct]

So take Sterling's BB series, front-ending a big expensive LFP bank.

If that is the only charge source allowed to touch the bank directly,

can custom-adjust the charge profile to whatever voltage you want

and "filter" the output from any DC charge source including a stock alternator, cheaper solar controller, old garage style mains charger, whatever.

No need to replace all the infrastructure.

Can transfer the LFP bank for use in a camper, another boat, off-grid home, no additional purchases needed.

Another benefit is the current limiting, prevent a thirsty AGM or especially LFP bank from damaging such old gear designed for low-CAR FLA by drawing higher amps than it can deliver safely.

Also allows DC conversion, say feeding a 24V bank from a 12V source or v/v.

Finally, can compensate for too much voltage drop over long wire runs, when the thick gauge otherwise required would be impractical.

[CatNewBee]

Originally Posted by BozSail

How does this differ from a regular battery charger, and what is its use on a boat, if any?

I'll try to answer your question.

Electricity flows (like water) from a higher potential point to a lower potential point along a wire (the potential difference is called voltage).

If you connect two batteries directly to each other, the battery with the higher voltage will drain into the battery of the lower voltage until both are on the same voltage level (assuming one is at 14V and the other at 12V, the 14V will discharge part of its energy and the 12V will charge until both have the same voltage - e.g. 12.5V)

Same applies to all charging sources. As long as a charger delivers a higher voltage than the battery has, there will be a charge current. When the battery has the same voltage as the charger, there will be no current any more.

A B2B charger is a one-way device, that connects to one source (e.g. battery), transforms the voltage to a higher level (step-up converter) to produce a voltage difference and to use the energy of the source battery to push a current into the target battery. If you connect the batteries directly, charging would stop when the batteries are equal, with a B2B charger you can drain the source battery and push all energy into the target battery. That is the concept behind. But a b2b charger can do more, it can for instance increase or limit the current going into the new battery and it can control the output voltage too, it can even implement charging stages and connect batteries of different chemistries or even voltages, so you can e.g. charge a 24V battery pack from a 12V battery.

It is a universal device, it works like a charge controller (in fact you can use in some circumstances a solar controller as B2B charger if you want e.g. to step down from 24V to 12V)

What are the use cases for a B2B charger?

1.) you have a dumb alternator, but want a controlled 3-stage charge regime

2.) you want to limit the (alternator) charge current (e.g. to protect the alternator from over-heating when using large lithium batteries)

3.) when your goal is to re-charge the house battery quickly when running on engine (e.g. in a RV, with a B2B charger you push most of the current into the house bat, and fewer energy into the start battery), so you boost the charge current.

4.) when you have different batteries with different voltages and want to transfer energy one way from one bank to another.

5.) to separate circuits efficiently and make sure you can control the direction of the energy transfer in contrast to a ACR, that connects the circuits directly.

The difference to a charger is, the charger uses AC shore power, a B2B charger uses 12V or 24V or... DC

[GILow]

Good post catnewbee.

I’ve got case number 4. Different voltage battery bank on house and engine.

My battery to battery charger allows me to pull power from the engine battery bank to charge the house battery bank if needed.

In my case the electronics have an easy job because they step voltage down from the engine’s 24 volts to the house 12 volts.

It also has multistage charging so changes the charging parameters based on the state of the house battery bank.

Not expensive and a nice fall back if my solar and wind generator could not keep up with demand. (Not all that likely)

[john61ct]

Note the "battery" to battery is a bit of a misnomer, used to distinguish those that handle energy input from any source, as opposed to those only designed to handle alternator output.

A battery is ***storage**, not usually an energy source. Alternator/genny, wind/solar and mains are your actual input to the DCDC charger.

These devices should never* be used to "drain the source battery and push all energy into the target battery"

Even if the input connection is the Starter batt terminals, the DC-DC charger is not transfering power from that battery to House, Starter is just acting as a buffer, and hardly any energy is actually flowing from (or to) the Starter, it just sits at Full all the time.

Note also a DC charger may just as often be used to step down voltage, from one nominal voltage to another (24 vs 12), or to protect a GEL or LFP bank from a poorly regulated or too-high voltage source.

Finally, they are not capable of boosting current, just regulate / convert the voltage, and in fact as stated may limit current, act as a bottleneck. But yes, when V is raised that will speed up charging compared to the lower V of the raw source, just saying nothing can "boost" overall power as in Watts.

If you want to pass larger currents than the max amps rating of the DCDC, say from a 180A alternator, units can be "stacked" in parallel.

_____
*rarely anyway, very limited circumstances where it would be worth putting up with the huge energy inefficiencies of going storage-to-storage

[CatNewBee]

Sometimes you can make sense to use a battery as source too, imagine you have a buffer battery for critical systems and you charge it plain from your large house bank, regardless of alternators or other sources..

[john61ct]

Yes, if the target batt is very small compared to source, and you have plenty of inputs so not worried about inefficiencies.

The scenario discussed in a couple "hybrid lead / LFP" threads here postulated this,

using the LFP bank to quickly absorb high amp ICE source output,

also getting Bulk stage up to a decent SoC on an expensive "Reserve" lead bank when current starts to taper down,

then after the ICE is stopped, do the "long tail" from LFP to lead at least a few days per week, when the solar isn't enough.

Basically reducing ICE runtime and extending the lead longevity, at a marginal cost in wear & tear to the LFP.

Most feel too complex, MS has repeatedly given a general thumbs down to the idea across several forums,

but it is one scenario where I think there is some practical justification for storage-to-storage transfer.

I can't really think of another, usually the smaller bank should just charge concurrently with the bigger one.

Maybe an electric dink, swap out its propulsion batts? was another thread about that.

[Q Xopa]

Originally Posted by john61ct

So take Sterling's BB series, front-ending a big expensive LFP bank.

If that is the only charge source allowed to touch the bank directly,

can custom-adjust the charge profile to whatever voltage you want

and "filter" the output from any DC charge source including a stock alternator, cheaper solar controller, old garage style mains charger, whatever.

No need to replace all the infrastructure.

Can transfer the LFP bank for use in a camper, another boat, off-grid home, no additional purchases needed.

Another benefit is the current limiting, prevent a thirsty AGM or especially LFP bank from damaging such old gear designed for low-CAR FLA by drawing higher amps than it can deliver safely.

Also allows DC conversion, say feeding a 24V bank from a 12V source or v/v.

Finally, can compensate for too much voltage drop over long wire runs, when the thick gauge otherwise required would be impractical.

Yes does those things. The limiting factor is they max out at 70A so in the case of a largish LFP bank your going to have to stack up a few to get a reasonable C charge rate. 600AH bank you'd ideally want 3 to give a 0.3C charge rate