Author Topic: How to charge a lower voltage battery from a higher voltage battery  (Read 148 times)

tecnodave

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To all users of this forum:


Using any MPPT controller to charge one battery from another battery bank is a bad idea for several reasons.

First off a MPPT controller is designed to get the maximum power from a source which has a limited power ability and will drop its voltage when under load. Solar panels do exactly that...the more load that you put on them the more that the voltage will drop.  Batteries do not do that if you are using them within their rated capacity. Also a utility driven power supply also should be able to regulate the output voltage within defined limits, so will not drop in voltage signifactly when used within their rated capacity.

So then batteries and power supplies are "stiff sources" of power, meaning that the voltage will not vary much when used within their rated capacity.

Solar panels are "soft sources" of power meaning that the voltage will have a much wider range of voltage than "stiff sources".

MPPT controllers will start with no load on the panels then quickly step by step load the panels down to a much lower voltage while measuring the voltage and amperage at each step. Calculations can then be made as to just what voltage that will produce "maximum power" . This happens very quickly and you will not be able to see this with any ordinary voltmeter. Even a high end voltmeter has a 25 millisecond read and calculate and write time. By observing the volts and amps on a oscilloscope you will see this voltage/amperage process. It happens very quickly, especially in high end MPPT controllers such as the Classic , Kid , and Outback units.

The term MPPT .....Maximum Power Point Tracking...means that the controller will do these sweeps to determine at which voltage that maximum power will be developed.

Batteries do not signifactly drop in voltage when used within their normal operating parameters.

So then a MPPT device is the wrong type of controller to use for this application.

Second point......the load output of the Kid controller is  a PWM switching device. It is good for non-inductive loads.....but the input of a MPPT controller does have capacitors and inductors.....it does generate "switching spikes".  These "switching spikes" are reflected back to the power source.

So if you are running one controller's output into the second controller the "switching spikes" of the second controller will be reflected back to the first controller which is not designed to accept this "noise".

Using one Kid's load output to drive the other Kid's input is exactly this problem.
The first Kid is is not really designed to run a load which is switching in nature thus highly inductive and the load output can be damaged by this.....the Second Kid is not designed to be powered by a "stiff source" and it's input section can be damaged by this.

So you are risking both controllers in doing this.

<edit>Added information: The Kid controller does its current measurement in its negative leads so if the two batteries share a common negative then the current  sensing will be bypassed....thanks Vic for reminding me.

Better to connect the second kid through a dropping resistor to the battery of the higher voltage system which will allow the voltage to "droop" under load thereby making the battery be more of a "soft source" of power.

There are better ways to achieve this, I use a DC powered battery charger to charge the lower voltage battery from the higher voltage.

Radio control model airplane hobbyists do exactly this to charge their LiPo batteries from their automobiles.

I have a iCharger i208 made by    Shenzhen Junsi Electronic Co., Ltd. located in Guangdong China
which can charge any type of chargeable battery, Lilo, LiPo, Lithium ferro  phosphate, Ni-cad, nickel metal hydride, lead acid.....whatever from a 12 volt or 24 volt source battery. It has a built in BMS and can charge 1-18 cell lead acid, 1-25 cell ni-cad or nickel metal hydride, 8 cell lilo, LiPo , and LFP batteries in balance mode with built in BMS and 10 cell LFP's in non balance mode.

It is a buck-boost converter. It can charge a 36 volt stack from a 12 volt or 24 volt battery or the optional external power supply which delivers 15 volts DC from 120 volts AC. 20 amps capacity to 350 watts maximum output.

I use this device to charge my power tool batteries, ni-cad and nickel metal hydride, and specially refitted power tools which were originally ni-cad or nickel metal hydride that I have converted to lithium using lilo batteries.  This charger does have a built in BMS (battery management system) which is required to properly charge any type of lithium batteries. Power tools that are factory equipped with lithium batteries cannot be charged in this way because they do not have user accessible BMS ports.

I hacked the battery cases of my Makita, DeWalt, and Panasonic power tools to include BMS ports when I converted from Nickel based batteries to modern 16550 type lilo's

I also use it to charge smaller AGM batteries, it's not really meant for AGM's as it does not have a proper AGM profile built in but this charger is fully programmable to any profile through it's built in USB communications port.

Details can be had from the manufacturer.

I bought mine from ProgressiveRC.  A model airplane shop found online....I will recommend them, quick service, best price. Good people.

Please visit Junsi at:            https://szjunsi.en.ecplaza.net
ProgressiveRC at:                http://www.progressiverc.com/

for more details

I think that I paid about $130.00 for the i208 and the optional external AC power supply....well worth the price. I also use it to charge up weak cells in lead acid batteries.

David




   <edit>. In proofreading I see that I have made several typo's and Apple iPad spelling is a thing of wonder, gotta go now, I'll be back here to correct errors and read feedback.    Tecno
« Last Edit: September 23, 2017, 07:44:55 AM by tecnodave »
#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 2 Suntech 280 24v AGM 105 a.h. Exeltech Inv.
#4 Kid/WBjr 2 Suntech 280 12v AGM 105 a.h.

unyalli

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #1 on: September 22, 2017, 07:32:33 PM »
Why not use a Brat?

- Jeff
"Problems can not be fixed at the same level of awareness that created them" ~ Einstein

CDN-VT

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #2 on: September 22, 2017, 08:08:40 PM »
So we have a Hydro mode that is a constant "stiff sources" of power !

Use a permanent magnet duel wound split stater  with forward swept magnets , (they spin easy for wind but are great for water wheels ) 3 phase the duel outputs (6 wires 0f 2 phases) run your distance & then a very large diode rectifier ( I use 3 Diode Rectifier per 3 phase & mix the phasing so the ripple is at min & this also adds a safety  factor that if we have  one diode die in a group of 3 , the dc should not feel any AC ripple , spike ) now all 3 diode DC outputs are paired & supply the KID with DC volts .
Up to 84Vdc  as I have seen ..
This is not soft power !!

VT
« Last Edit: September 23, 2017, 10:31:52 PM by CDN-VT »
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c +30c max  4 strings in 2 in Series for 24v Classic 150 -1020 Ah  Freezers & fridges ~~~ Second Array same panels of 3sx3 parallel for 24 V Classic 150 -440 Ah Outback Barns & out blds.
48Vdc almost done.

tecnodave

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #3 on: September 22, 2017, 11:47:21 PM »


Why not just use a Brat?

The Brat is a solar controller of the PWM type, the iCharger is a buck/boost battery charger. Different device entirely. I can charge a 36 volt set batteries from a 12 volt source. I don't know of any solar controller that can do that. I do have quite a few battery power tools that outlived their batteries and have been repowered with LiPo batteries. I cannot buy new batteries for my 18 volt NiMh power tools so I repowered them with commonly available 16650 LiPo batteries.They are much lighter and have much more power and run longer and charge faster.

The Brat seems to be a great unit and is in the same price range as the iCharger, but does not have anywhere the capability of the iCharger. The Brat can only charge certain types and you need an external BMS to charge LiPo batteries. The iCharger has a BMS built in and is fully programmable. If you can invent a new battery type you can program the iCharger to charge it.

I do not have a Brat but based on my expereince with Midnite that is the controller that I recommend for small solar systems such as a few solar panels on a motorhome.

I see the Brat advertised at NAWS for $89.00   I paid $109.00 for my iCharger from ProgressiveRC the AC power source was another $39.00 or so.

The Brat is rated at 30 amps and 12 or 24 volts, the iCharger i208 is rated at 20 amps at any voltage from 1.2 volts to 36 volts......totally different devices....what solar controller can charge one nickel hydride battery?  .....1.2 volts. I sure have not heard of one.

The iCharger fits in my pocket, it's larger than the Brat.

It also has 16 megabytes of data logging and can export data through its data port.

It also can renew nicad batteries to full capacity and many more neat features, discharge LiPo batteries to a storage state......you cannot store LiPo batteries at full charge.   It is just the best small battery charger that I have found.

The iCharger i106 is far more common but it is a 6 cell charger rated at 10 amps  with built in BMS. That is the model that I see most commonly used in the RC model airplane. Bit cheaper but does not have the capability to run off my 24 volt solar system.

Bottom line,  very portable, very capable charger, conditioner, discharger.

Read the manual online.......it's a very capable unit at a very reasonable price and an absolute must if you have small LiPo , LiLo , or LFP battery packs......even eBikes

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 2 Suntech 280 24v AGM 105 a.h. Exeltech Inv.
#4 Kid/WBjr 2 Suntech 280 12v AGM 105 a.h.

tecnodave

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #4 on: September 23, 2017, 12:21:42 AM »
If you really want to use a Kid to charge the lower voltage battery bank at the minimum,

Power the Kid directly from the higher voltage bank,

Do not connect the negative of the lower voltage bank to the negative of the higher voltage bank,this will protect the current sensing elements of the kid. The negative lead from the higher voltage battery goes to the input of the Kid only

Use a resistor between the positive of the higher voltage bank and the positive input of the Kid...maybe 25 ohms or so and 25 watt .....Not Wire Wound type......that would make it an inductor!
This will "soften" the power a bit and allow the Kid to sweep to find the best voltage to operate at.
Set the current limit at a reasonable level

Maybe the voltage drop across the resistor should be about 30% of the difference between the voltage of the two batteries.

This should protect the Kid.  BUT  I still do not think that it is a good idea, wrong tool for the job!

David

<edited for clarity>
« Last Edit: September 23, 2017, 07:55:57 AM by tecnodave »
#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 2 Suntech 280 24v AGM 105 a.h. Exeltech Inv.
#4 Kid/WBjr 2 Suntech 280 12v AGM 105 a.h.

russ_drinkwater

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #5 on: September 23, 2017, 12:40:48 AM »
I will scratch that off a list of things to do!
Abundance of solar panels makes solar my best option, lol.
Standalone. 20 Hyundai x 220 watts panels, 2 x classic 150's, Latronics 24 volt 3kw inverter, Whiz bang Jnr, 12 Rolls surrete  4KS 25P  batteries and WBJ.
Grid tie feed-in, 12.5 kw in 3 arrays generating 50 kws per day average. Solar river grid tie inverters

tecnodave

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #6 on: September 23, 2017, 07:35:21 AM »
Russ_drinkwater

I totally agree with that statement, I have piles of good panels that were bought for under $50 each....even 200-250 watt ones, I have only 4 really good charge controllers (all Midnite) and about a dozen useful ones (Trace and Morningstar PWM) , a growing pile of burnt and unreliable ones, including Morningstar MPPT and BZ MPPT and lots of Chinese junk including Tracer MPPT (not to be confused with Trace)  I will buy non operational solar systems and remove them on the gamble that I will get some good gear out of the pile. Really good controllers are not found surplus or used cheap. Panels are dirt cheap here.

David
« Last Edit: September 23, 2017, 08:00:49 AM by tecnodave »
#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 2 Suntech 280 24v AGM 105 a.h. Exeltech Inv.
#4 Kid/WBjr 2 Suntech 280 12v AGM 105 a.h.

CDN-VT

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #7 on: September 23, 2017, 10:38:23 PM »
I agree Guys
Hydro is a totally different math than solar & weather dependent .


I haven't done any wind (as in fan on a pole )
My wife says different !!!!

VT
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c +30c max  4 strings in 2 in Series for 24v Classic 150 -1020 Ah  Freezers & fridges ~~~ Second Array same panels of 3sx3 parallel for 24 V Classic 150 -440 Ah Outback Barns & out blds.
48Vdc almost done.

russ_drinkwater

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #8 on: September 24, 2017, 12:24:21 AM »
Yeah, I got IBS and I still love $50 solar panels!
Wind turbines are just a play thing, but I freely admit the money I have spent on them and hubs, blades etc etc from windy nation would have been better spent on 2 more grid tie inverters and 30 more 2nd hand panels.
I fiddle for the enjoyment of discovery!
Once you stop learning, exploring and thinking of projects you are for all intensive purposes dead!
My idea on life anyways.
Standalone. 20 Hyundai x 220 watts panels, 2 x classic 150's, Latronics 24 volt 3kw inverter, Whiz bang Jnr, 12 Rolls surrete  4KS 25P  batteries and WBJ.
Grid tie feed-in, 12.5 kw in 3 arrays generating 50 kws per day average. Solar river grid tie inverters

tecnodave

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #9 on: September 24, 2017, 12:33:59 AM »
Russ_drinkwater,


"Once you stop learning, exploring and thinking of projects you are for all intensive purposes dead!"


 +++ my thinking exactly!

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 2 Suntech 280 24v AGM 105 a.h. Exeltech Inv.
#4 Kid/WBjr 2 Suntech 280 12v AGM 105 a.h.

russ_drinkwater

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #10 on: September 24, 2017, 05:20:15 PM »
Every day is an adventure of discovery for me Dave! Cheers. ;D 8) :P
Standalone. 20 Hyundai x 220 watts panels, 2 x classic 150's, Latronics 24 volt 3kw inverter, Whiz bang Jnr, 12 Rolls surrete  4KS 25P  batteries and WBJ.
Grid tie feed-in, 12.5 kw in 3 arrays generating 50 kws per day average. Solar river grid tie inverters

tecnodave

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Re: How to charge a lower voltage battery from a higher voltage battery
« Reply #11 on: September 27, 2017, 08:42:02 PM »


Back in the 80's Buckminster Fuller stated "Wind power is Solar power 99 to 1". With the advent of cheap solar power that is no longer true.

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 2 Suntech 280 24v AGM 105 a.h. Exeltech Inv.
#4 Kid/WBjr 2 Suntech 280 12v AGM 105 a.h.