Using a Kidd to charge one battery (12V) from another battery (48V)

Started by sailingcinderella, March 08, 2019, 02:52:17 PM

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sailingcinderella

Hey guys,

Cinderella here, we got back to the boat in Tahiti and WHAT A MESS!!

It turns out leaving a boat in the tropics in the rainy season is a moldy affair.

Anyway, we came back to find our motor battery bank was toast. So we had to buy new batteries and have them shipped from India. We will see how long that takes... At least Tahiti is a fun place to be stuck.

We will now have a nice new 48V battery bank of carbon foam AGM batteries.

I'd like to use that bank to charge my 12V bank and thus increase my house battery capacity. It's also convenient because the Kidd currently charging my 12v bank can't handle all of the 690 watts the panels make when charging @ 12V.

What I'm considering is connecting the panel side of a Kidd to a 48V battery bank and the battery side to a 12V battery bank. Can it handle it?? Will I fry something?

That way the second Kidd can handle charging the 48V bank from the panels and I can utilize all of the power the panels make without anymore equipment.

Thanks guys!

Vic

Hi Cinderella,

Sorry to hear of the issues with your battery,  boat and the mold.

Generally,  using a CC to charge a lower voltage battery from another battery is not a good idea.

And  the PV input to the KID,  that normally connects to the PV array needs its PV negative terminal to "float",   and not be connected to the negative terminals of either battery  (or to any other negative),  so,  it would seem that this could be a difficult situation,   depending on just how the 48 volt battery bank's negative is handled.

This question has been asked  here,   before,  the following is taken from a Post in another Thread,  from a Senior Tech Support person at MidNite,

"   ...   The issue is the stiff source [of current from the source battery]   and the fact it is not current limited like a pv module, You would want to use Hydro mode and slowly drop the fixed MPPT voltage down until you get some current flow. Also make SURE you have the current limit set to a lower value to start and get things working. Last thing I could suggest is a large 1 or 2 ohm resistor in series in the [48]  volt positive line to soften it some.'

Also be aware this will void your warranty most likely

FNG"

The above from FNG is another reason to not try this.
FWIW.   Good Luck!   Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

tecnodave

Sailingcinderella,

Please read this post for more detail on doing what you suggest before proceeding with this.

http://midniteftp.com/forum/index.php?topic=3841.0

I wrote several articles in there,

I will suggest a better way of doing that. Use a DC-DC converter. I am using Samlex America buck converters for this application. The buck converter has a fixed output and can "float" a lower voltage battery but does not have the proper 3 stage program to properly charge a lead acid battery.
I have several Samlex America SDC-30 buck converters that are 20-30 volts in and regulated 13.6 volts out which is a proper float voltage for lead acid batteries. One app that I am using this for is my burglar alarm systems which are 12 volt DC . I use a small 12 a.h. local battery charged by a DC-DC converter of 6 amps capacity, which in turn is powered by my main banks of my solar system. I have double redundancy there as the local battery is the 3rd power source, DC buck converter will power even if the local battery fails and main power is from 24 volt banks.

One thing to be aware of is that all DC-DC switching devices switch one or the other leg of the supply source so some cannot be connected togather. The Samlex units are common negative so the negative of the source battery can be connected to the negative of the charged battery.

In the above mentioned thread  I speak of the Junsi charger.....the iCharger i208B....be aware Junsi Chargers are NOT COMMON NEGATIVE.....if you connect that charger between two common negative battery systems you will let out the magic smoke!  The FET's will blow instantly. I knew to be aware of that but I made a mistake in my hookup and smoked the unit.

David
#1 Classic 150 12 x Sharp NE-170, 2S6P, 24volt L-16 Rolls-Surette S-530, MS4024 & Cotek ,  C-40 dirv.cont. for hot water
#2 Classic 150 12 x Sharp NE-170, 2S6P, 24 volt L-16 Interstate,Brutus Inv.
#3 Kid/WBjr 4/6 Sanyo 200 watt multilayer 4/6 P
#4 Kid/WBjr 4/6 Sanyo 200 watt multilayer 2S 2/3 P

ClassicCrazy

The other thing to consider doing it to change your 12v system to 24v - then the same Kid will charge twice as much as it was doing at 12v. ( 30 amps at 12v =360 watts  while 30 amps at 24v  = 720 watts )  You would have to do the string calculator for your particular PV panels to see how your configuration would be. You can then get a Samlex SDC-23 or other DC to DC converter to change the 24v to 12v - easy to wire that into your existing 12v system .

Larry
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable

Vic

Should add,   that as Larry mentioned use the KID String Sizing Tool to make sure that your PV s are configured to have enough voltage to allow the KID to find the best operating point to charge a 48 V battery.

Often strings that are configured for a 12 V battery will not have sufficient  voltage to allow an MPPT  Charge Controller to work well,  or,  perhaps even at all.

FWIW,  Vic
Off Grid - Sys 1: 2ea SW+ 5548, Surrette 4KS25 1280 AH, 5.25 KW PV, Classic 150,WB, Beta Barcelona, Beta KID
Sys 2: SW+ 5548s, 4KS25s, 5.88 KW PV, 2 ea. Classic 150, WB, HB CC-needs remote Monitoring/Control, site=remote.
 MN Bkrs/Bxs/Combiners. Thanks MN for Great Products/Svc/Support&This Forum!!

unyalli

Samlex is quality, I've used them with no complaint. Victron also make DC to DC down converters and some of them have adjustable 12v output.
https://www.victronenergy.com/dc-dc-converters/orion-dc-dc-converters-12v-24v-48v-110v
As previously stated use the kid to harvest energy from solar. For converting higher DC voltages to lower use devices designed to do so.
Agree with classiccrazy, let the kid charge a 24 volt battery then let it dc/dc down convert your 12v system. Triple backup.
- Jeff

sailingcinderella

Thanks guys!

Sounds like I’ll need to just pick up a DC-DC converter and I’ll be off to the races.

If only those were easy to find in Tahiti...

boB

Quote from: sailingcinderella on March 11, 2019, 11:05:11 PM
Thanks guys!

Sounds like I’ll need to just pick up a DC-DC converter and I’ll be off to the races.

If only those were easy to find in Tahiti...

Yeah, no radio shacks or surplus stores or the like in Tahiti.  However, there are some hams down there !  Maybe they could help ?

Does Amazon deliver down there ?   Hmmmm....

boB
K7IQ 🌛  He/She/Me