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Old 20-01-2015, 18:36   #1
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charging a small battery from a larger battery

Hi,
I'm considering options for isolating the charging of my start battery from my house battery, due to the facts that a) the start battery (tiny 350cca) is much smaller in capacity b) always nearly full unlike the house battery (130A/h).
My engine has about a 60amp alternator, and a Nextstep-12 smart regulator.
The reg has an output to drive the combining relay. The alternator output is connected to the house battery, and the combiner connects the start battery once the voltage comes up to the trigger point (much the same as a VSR works).
The charging mode (bulk/absorbtion/float) cant possibly suit both batteries at the same time, and the temp probe is only monitoring the house battery, so my idea is to charge the house using alternator and smart reg, and charge the start batt using a DC-DC charger. These seem to be relatively uncommon, but there's a few around, like the C-TEK D250S Dual, the Powerstream PST-BC1212-15, etc.
What I'm wondering is; why not just use a voltage booster Booster 12/24V 8 32V to 9 46V DC Step up Voltage Converter 150W Notebook Mobile Regulated Power Supply Module #090438-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group (a la Aliexpress), and a cheapish 10A PWM solar regulator with 3-stage charging.
Seems it would achieve the same end result, for a lot cheaper? I guess if the voltage booster is rated at or below the solar regulator, they should play nice together (or would the solar reg try and pull more than the booster can safely supply - need to be current limited?)?
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Old 20-01-2015, 19:27   #2
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Re: charging a small battery from a larger battery

I don't see any problem with the system you already have. If the starter battery is in a higher state of charge than the house batteries, when the combiner activates the majority of the charge will go to the battery with the lowest voltage IE your house bank, so you won't be grossly overcharging or burning up your start battery.

Also, even the bulk charge voltage should not be so high as to damage the battery anyway. What's it set to?
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Old 20-01-2015, 19:46   #3
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Re: charging a small battery from a larger battery

Or you can remove relay completely and use Xantrex Echocharge:

"Xantrex’ digital echo~charge (part#82-0123-01) is designed to charge auxiliary or starting batteries from an inverter/charger or other charging source with limited voltage drop. The Xantrex echo~charge detects when the house battery bank is being charged and directs a portion of the charge current to auxiliary or starting batteries. The maximum charger current offered by echo~charge is limited to 15 amps.
15 A maximum charge current
Automatically switches on/off without affecting the in-house battery bank or over-charging
Utilizes excess current from the primary charging source"
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Old 20-01-2015, 21:02   #4
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Re: charging a small battery from a larger battery

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Originally Posted by BambooSailor View Post
Or you can remove relay completely and use Xantrex Echocharge:

"Xantrex’ digital echo~charge (part#82-0123-01) is designed to charge auxiliary or starting batteries from an inverter/charger or other charging source with limited voltage drop. The Xantrex echo~charge detects when the house battery bank is being charged and directs a portion of the charge current to auxiliary or starting batteries. The maximum charger current offered by echo~charge is limited to 15 amps.
15 A maximum charge current
Automatically switches on/off without affecting the in-house battery bank or over-charging
Utilizes excess current from the primary charging source"
Exactly! This is precisely what the EchoCharge and the Balmar DuoCharge voltage follower devices are designed for.

Simple, clean, you don't have to switch anything or worry about large amperage transfers which could possibly occur with VSRs.

Mine has worked flawlessly for almost 10 years now, operating 24/7. So have those I have installed on customer's boats.

Bill
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Old 21-01-2015, 17:57   #5
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Re: charging a small battery from a larger battery

Thanks for the replies guys.

I'm not sure what the absorbtion rate is set to, but I thought that typically the voltages used on smart regs would lightly gas a lead acid, to ram in charge at max possible acceptance rate?
I should have mentioned the dinky start batt is a 'maintenance free' lead/calcium battery, so topping up the electrolyte is not possible.
However I've since read that these batteries accept a slightly higher voltage than lead/antimony batteries, so maybe what lightly gasses the big house batt will be fine for the start batt? I'll try and meter the voltage at the batteries this weekend.

Yes, I'd heard of the echo charge also - seems they would work much the same way as the DC-DC chargers I mentioned, and prevent overcharging. Nice and compact too, which is good. The stop charging feature seems unnecessary as a good charger would go in to float anyway. The thinking was to use the charge relay to cut supply power to the charger, so it would not drain the house battery when engine is off.

All this said I should mention I'm on a tight budget and hence the pondering over my 'creative' low cost solution, and wondering if it would work?
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Old 21-01-2015, 19:11   #6
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Re: charging a small battery from a larger battery

Don't waste your energy, time, and $$ on a "low-cost solution".

The EchoCharge is exactly what you need. It's small, reliable, requires no maintenance, and won't cook your maintenance-free battery. It stops doing anything when the voltage on the house bank drops below about 12.9-13.0 VDC.

Install it and forget it.

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Old 22-01-2015, 08:03   #7
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Re: charging a small battery from a larger battery

I think you are fine the way you have things now. We use VSR's to bring our small WalMart automotive start batteries into charge with the large house bank with no problems. Have never had to add electrolyte to the start batteries, even though they experience daily full-charge regimes from a 120A charger and solar to 14.8V through absorption on the house bank.

We had to replace our start batteries after 14yrs - they lasted through three sets of house banks.

I think you are over-thinking this. Why spend money where a problem doesn't exist?

Mark
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Old 22-01-2015, 08:36   #8
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Re: charging a small battery from a larger battery

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Originally Posted by colemj View Post
I think you are fine the way you have things now. We use VSR's to bring our small WalMart automotive start batteries into charge with the large house bank with no problems. Have never had to add electrolyte to the start batteries, even though they experience daily full-charge regimes from a 120A charger and solar to 14.8V through absorption on the house bank.

We had to replace our start batteries after 14yrs - they lasted through three sets of house banks.

I think you are over-thinking this. Why spend money where a problem doesn't exist?

Mark

Bingo!!! Save your money....
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Old 22-01-2015, 09:42   #9
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Re: charging a small battery from a larger battery

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Originally Posted by Maine Sail View Post
Bingo!!! Save your money....
Yes. If you're on a tight budget, this seems to be working so just leave it. Very unlikely to cook your start battery.

If it ain't broke, don't fix it.
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Old 22-01-2015, 09:50   #10
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Re: charging a small battery from a larger battery

I gree.

Overcharging with Combiners or ACRs The MYTH:
Overcharging Batteries with a Combiner or ACR

A combiner is just fine, you won't "overcharge" because you simply can't.

Stick with what you have.
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Old 22-01-2015, 09:50   #11
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Re: charging a small battery from a larger battery

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Originally Posted by skipmac View Post
Very unlikely to cook your start battery.
I know this sounds a bit pedantic, but it is not only unlikely to cook the battery, it will not cook the battery. Unless, of course, the charging system itself is set up to be bad for the battery type.

I only make this distinction to help clear up confusion about combining batteries, and not leave open interpretations - not to nitpick your post.

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Old 22-01-2015, 10:00   #12
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Re: charging a small battery from a larger battery

To give some actual data on what is happening with a VSR, right now we have our charger on. The combined batteries are currently at 14.6V with the house bank taking 85A and the start battery taking 0.2A.

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Old 22-01-2015, 10:45   #13
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Re: charging a small battery from a larger battery

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Originally Posted by colemj View Post
I know this sounds a bit pedantic, but it is not only unlikely to cook the battery, it will not cook the battery. Unless, of course, the charging system itself is set up to be bad for the battery type.

I only make this distinction to help clear up confusion about combining batteries, and not leave open interpretations - not to nitpick your post.

Mark
Good thought.
- As long as all the batteries are of the same type (ie: old fashioned flooded cells), shouldn't be a problem.
- If you have say a flooded cell starting battery and an AMG house bank, still unlikely to be issue but you may want to look at the charge voltages provided to make sure they are OK for the more senstive battery type.
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Old 22-01-2015, 10:56   #14
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Re: charging a small battery from a larger battery

How about coupling non-lead acid batteries to a lead acid (of whatever flavor)? I use a Lithium FePO4 as my outboard's starter battery (it's a Lehr 15 hp propane outboard) and I'd dearly love to increase, slightly, the capacity for an electronics package for my "scout boat".
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Old 26-01-2015, 17:40   #15
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Re: charging a small battery from a larger battery

Thanks for the further replies. Good to hear others have not had issues after years with similar setups. Will leave all as is - suits me well!
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