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Old 02-03-2017, 13:08   #1
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Battery-to-battery charger (independant of source charge voltage)

I am looking for a battery-to-battery charger (12V to 12V), which will operate regardless of the presence of a charging voltage on source battery. Most of the units will only engage when they sense 13+V on the source battery. There is a Sterling unit which can be placed in manual mode, but it still only engages at 12.4V and above.

Thus I am looking for a unit which will take a nominal 12V and boost it to 13.8/etc to charge a separate battery bank.

The use case is I have a large house battery bank, which is re-charged say daily. I also have an emergency battery bank, to which are connected radios and navigation equipment. The charger (currently a Hopkins Engager Break-Away System) keeps the emergency bank charged. Thus if the house bank is lost (it is below decks) the emergency bank in the deck house keeps the critical equipment functional with no switch over time.


Thanks,

Allan.
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Old 02-03-2017, 15:07   #2
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Re: Battery-to-battery charger (independant of source charge voltage)

Key point missing here: how big amperage you looking for?

Very few units are DESIGNED to do this, since it is very inefficient.

My use case is a huge high-amp-fast-charging bank A that never needs to get to full (LiFePO4), and a much smaller backup/reserve/starter bank B made of lead, which must be 100% topped up to full for longevity.

When using fossil fuel for recharging only, using the DCDC charger this way will allow me to both cut runtime by many hours AND extend bank B's lifetime to infinity and beyond, as opposed to replacing it every year or two.

My search is made easier in that bank A LFP sits at higher voltage than lead bank B anyway. So I can re-purpose any DCDC charger, as long as it only uses a low enough voltage as its ONLY switch-on criterion.

The ability to go from quite low to higher will be rare.

Tell me, if you're charging every day, why aren't you just charging both banks at the same time?

And what batteries are in your two banks? Ideally specific model, at least brand, otherwise just general type.
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Old 02-03-2017, 15:23   #3
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Re: Battery-to-battery charger (independant of source charge voltage)

Just came across this, pretty exciting IMO: https://sterling-power.com/products/...harging-module
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Old 02-03-2017, 21:51   #4
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Re: Battery-to-battery charger (independant of source charge voltage)

the victron orions do this but they are only single stage. IE you set the output voltage to what you want say 13.5v. and they take 8-18v input so always working.

https://www.victronenergy.com/dc-dc-...rters-isolated
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Old 03-03-2017, 04:24   #5
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Re: Battery-to-battery charger (independant of source charge voltage)

Perhaps an adjustable high-voltage cutoff would automate preventing overcharge, but if the target bank is lead, you really want a staged charging algorithm to get it through a solid CV absorption cycle.

Since there are large inefficiencies, floating for extended periods would be wasteful, so a shutoff timer would be useful in any case.
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Old 03-03-2017, 04:40   #6
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Re: Battery-to-battery charger (independant of source charge voltage)

Nothing is going to "take 12v and boost it to 13.8v".
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Old 03-03-2017, 04:47   #7
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Re: Battery-to-battery charger (independant of source charge voltage)

That's just silly.

That's exactly what battery-to-battery chargers are for!

Generic term is DC converter with boost capability, thousands of eBay items alone will do that.

Getting an automated high-amp proper staged battery charger with a low enough "start trigger" setpoint is the challenging part.
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Old 03-03-2017, 04:47   #8
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Re: Battery-to-battery charger (independant of source charge voltage)

The Mastervolt Magic series will act as a 12v battery to 12v battery charger in the way you require.
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Old 03-03-2017, 04:50   #9
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Re: Battery-to-battery charger (independant of source charge voltage)

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Originally Posted by noelex 77 View Post
The Mastervolt Magic series will act as a 12v battery to 12v battery charger in the way you require.
Do you know what voltage it is set for a "start trigger"?

Or can you override "force start" manually?
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Old 03-03-2017, 05:02   #10
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Re: Battery-to-battery charger (independant of source charge voltage)

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Originally Posted by john61ct View Post
Do you know what voltage it is set for a "start trigger"?
The handbook is here:
http://www.mastervolt.com.au/product...-12-12-20/pdf/

It will function a proper 3 stage 12v to 12v battery charger. From the specifications it needs a minium of 12.0v to function in this mode.

I would presume it will maintain float voltage while the input is 12.0v or above and will likely start a new bulk,abs,float cycle if turned off and on (providing the input voltage is 12.0v or above). In other words similar too a normal AC battery charger (although most will start a new cycle if the battery voltage falls below a preset value and some will start a new cycle every 24 hours), but this is only my interpretation from reading the handbook.

A "trigger voltage" is more applicable to a VSR. The unit is not trying to perform this function.

An email to Mastervolt would be sensible.
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Old 03-03-2017, 05:45   #11
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Re: Battery-to-battery charger (independant of source charge voltage)

I would presume the unit stops delivering a charge when the source bank voltage falls below a certain setpoint, and then restart at another (and/or with delays built in), without manual intervention needed.

That's all I meant.

Functionally, once a more intelligent solution like a DCDC charger is in place, a cruder one like a charging ACR/VSR is usually no longer needed.
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Old 03-03-2017, 05:49   #12
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Re: Battery-to-battery charger (independant of source charge voltage)

Does the engine / genny charging go to the house bank and the Hopkins is powered from what?

If so why not just wire in a VSR from the house to the emergency bank. When the house bank is charging the VSR connects above 13.7v and disconnects below 12.8v so protecting the emergency batteries. If it all went pear shaped the emergency bank should run those electrics for a few while even with the house bank low.

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Old 03-03-2017, 09:36   #13
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Re: Battery-to-battery charger (independant of source charge voltage)

More work, but on work boat systems they will sometimes run two feeds to the helm, one from the pilothouse battery (or converter on larger ships) and the other from the main battery bank, they will either have a manual change over or a schottky diode set up to prevent back charging. You could use a system like that plus a power conditioner. This will end up being more hardware plus if you use the diodes you will have some voltage loss. But it would be a fairly robust system.
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Old 03-03-2017, 10:36   #14
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Re: Battery-to-battery charger (independant of source charge voltage)

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Originally Posted by clownfishsydney View Post
Nothing is going to "take 12v and boost it to 13.8v".
Of course you can, the electrical component that does this is called a transformer which is two sets of coils that are intertwined but insulated from one another. You can buy or make them to step up/down voltage by any ratio you want. If you need it to be variable (i.e. you need 13.6-13.8 volts no matter what the source charge is, the transformer just need a connector that slides up or down the coils until the output is at the desired voltage.

Now, the WATTS won't change (except for a little bit of loss to heat), but as the volts go up, the amps will go down, and as the volts go down the amps will go up.
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Old 03-03-2017, 13:50   #15
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Re: Battery-to-battery charger (independant of source charge voltage)

I fail to understand your problem. Many chargers are available that will keep both banks charged. if you can charge one bank daily why not charge both? As to a seamless cutover to the 'emergency bank', sounds dangerous to me. If I'm forced to emergency power I want to know it. I think many monitors coupled with an alarm can tell you your house bank is low so you can manually switch to your back up. Why would you want the inefficiency of moving power from one battery to another?
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