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Balancing charging of sixteen LiFePO4 cells

BruceS

Jun 25, 2014
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A brief description first.
I've got 16 LiFepo4 cells all connected in series for (nominal) 48V.
I have a monitor that will activate an alarm or trigger a relay on EACH cell. (pos & neg)
I'm thinking of having the relay close contacts on high voltage (preset at 3.6V DC) and to connect another set of heavier wires from the high cell to a 100w globe or similar to drag down that cell while at the same time disconnecting the solar charger. It will be run through a timer that controls the relay. I'm thinking a 3 minute setting initially before it reverts to globe off & charging restarting.

OK....... my question is whether I can run all of the heavier wires through diodes so I only need one globe. It is very unlikely to ever have 2 at HV at the same time. Remember each cell has two wires from it. Not a common earth.
For those that don't know.... these cells won't equalize or 'self balance'.
By keeping the high cells down the charger will control things by itself nicely.
Thanks for your input!
 

Harald Kapp

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Welcome Bruce.

You would need dpst relays to switch both contacts of teh globe to the cell you wish to discharge. Diodes are not sufficient.

I think a much better way is an electronic charge balancer. Instead of dissipating excess energy, it can route part of the charge current around a full cell and use it to charge other cells. Google "charge balancing ic", these ics are made by several manufacturers.
 

BruceS

Jun 25, 2014
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I wonder if anyone can help me translate a Chinglish instruction please?
I have a small assembled kit that can output either to an alarm or relay on LV or HV but it says the following in the instructions.

  • Overvoltage and undervoltage signal independent application, control two relays separately. When power on, relay connect. When out of the upper and lower limit set value, relay disconnect. Relays can be replaced with the other loads. If you need reverse control, please add a reverse transistor.
By 'reverse control' I assume they mean relay opens rather than closes?
What is a 'reverse transistor'? Any links to an example welcome.
 

KrisBlueNZ

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It sounds like there are two separate relays which are both normally energised, so their contacts are normally closed. When an undervoltage or overvoltage occurs, the corresponding relay de-energises, i.e. its contact opens.

This is called a "fail safe" type of alarm, because the alarm annunciator expects its control circuit to be closed to turn the alarm OFF; if the circuit is broken because of a loose connection, broken wire, etc, the alarm annunciator activates, and you find out immediately. With the alternative arrangement, if the connection to the annunciator is broken, you don't know about it, and the annunciator can never activate.

You can combine the two alarms (so the annunciator activates on either undervoltage or overvoltage) by connecting the contacts in series. If either contact opens, the circuit is broken.

At some point though, you have to convert the normally closed loop into a normally open circuit that energises the annunciator (siren, bell, or whatever) by completing the circuit between that annunciator and the power supply when it detects that the control circuit goes open.

This conversion can be done with a transistor or a MOSFET. It depends on what you're using as your annunciator. What is your annunciator? How much current does it draw? How much voltage does it use?
 

BruceS

Jun 25, 2014
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Thankyou Kris, ... I should have included a link to it.
http://www.ebay.com.au/itm/171136171020?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
I've done a little bench testing so far.
One think I've discovered is that it appears as if the HV & LV alarm/relay outputs are reversed from what is indicated on the PCB. I used 12V to power the board & was 'testing' a slightly flat 9V smoke alarm battery.
The HV output (on the LV terminal!) was 3.5V using the 'common' neg on that outlet side.
I'm not sure if it gets the output voltage from the supply or the test....... would think from the supply?
With output voltage that low I suspect it's designed more for Arduino or similar but should operate a SSR?
Handy little gadget if it does what is says it does.
By the way they appear to have made a mistake about the 2 wire/3 wire bit.... Both indicate 30VDC.
I never tested current draw.... yet.
 

KrisBlueNZ

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OK, the description is pretty clear: "Upper and lower limit alarm output signal is high level normally, if exceed the set value is low level."

The HV and LV outputs are both high (+3.3V I think, relative to the circuit's 0V rail) if the voltage is within range. If the voltage goes out of range, one of those outputs goes low.

So you want to control an SSR? Do you want it to activate when the voltage is out of range? Or do you want it to be closed normally and go open when the voltage is out of range?

What do you want the SSR to control? How much current and voltage does it need to switch? Do you have a particular SSR in mind?
 

BruceS

Jun 25, 2014
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Unfortunately most of this is way too complicated for me to understand but I'll have a go.
I'm looking at an alternative way to control voltage of batteries as a secondary safety backup.
The batteries are LiFePO4 type. 16 cells in series = nom 48V.
These batteries do not equalize. I'm looking at a way of achieving 2 things.
1. Reduce the voltage in an cells that go high. (reach a set point)
2. Switch off all charging if a cell goes low. (reach a set point)
The cells/ battery is charged from solar and grid power/generator. (at various times depending where the boat is)
So when a cell reaches HV, I'd like to switch on a dump load on that cell as well as switch of ALL charging for a set time. (3 mins initially for testing purposes) At the end of that time normal setup resumes. Once total battery voltage reaches voltage set in combo unit (inverter/solar charger/battery charger) the combo unit will restrict charging as normal charger does.
When a cell or complete battery drops to a LV set point the batteries will be totally disconnected from any further drain/useage until manually inspected/turned back on.
I know I've had suggestions to look at commercial battery equalizers but from other's testing/use they are not capable of controlling such big battery cells. They are more for remote controlled cars/planes/boats etc.
Milli amps are not enough to dump, I need several Amps at least. Maybe 100w globes. How much dump will I get with 3.6VDC on a 100w 12V globe?
Gadgets looked at/bought.
The units as I linked to above.
These timer relays.
http://www.ebay.com.au/itm/200973250865?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
I've got access to a 3vDC control 40AAC SSR to cut AC.
100A continuous duty magnetic 12VDC control relay for battery isolation.
150A continuous duty Gigacvac 12vDC control contactor for solar isolation.
Looks like I need some 3VDC control 12VDC relays for stepping up voltage? lol.
It's more a hobby/challenge!!
Thanks Kris...
 

KrisBlueNZ

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Ok Bruce, I've merged your new thread into your existing thread on the topic of balancing these sixteen cells, and given it a useful name.

I have no experience with battery balancing. Other Electronics Point folks do, and I hope they can offer some advice.

Your requirements seem to be unusual in two ways: there are sixteen cells (instead of the more common three, four or five), and the currents are quite high. Also, your chemistry may not be standard - I don't know.

Can you tell us the manufacturer and part number of the cells, and give us a link to the data sheet. In particular I'm interested in the capacity and the chemistry of the cells.

I'm also interested in your charging source, and your loads. You mentioned a boat. Do you have solar charging? What are the loads? Background information will all help us understand what you're aiming to do.

How are you charging the battery? Do you have a manufacturer and part number for the charger? What algorithm does it use? How will it respond to having its mains supply cut off and restored while it's part-way through the charging process?

What is the charge current? What is the maximum discharge current?

Re your question about 12V 100W incandescent bulbs. Their resistance at 12V can be calculated as R = V2 / P = 144 / 100 = 1.44 ohms. At only 3.6V, the filament will be colder than it would be at 12V, so its resistance will be lower. Perhaps 1.2 ohms. So current can be calculated as I = V / R = 3.6 / 1.2 = 3A. This number is only approximate.
 

BruceS

Jun 25, 2014
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Kris I appreciate you comments. I'll answer in amongst your post in colour if I can!
Can you tell us the manufacturer and part number of the cells, and give us a link to the data sheet. In particular I'm

interested in the capacity and the chemistry of the cells.

The battery cells are 16 X 100AHr seen here.
http://en.winston-battery.com/index.php/products/power-battery/item/wb-lyp100aha?category_id=176


I'm also interested in your charging source,
I have an MPPSolar 'Combo' unit seen here.
http://www.ebay.com.au/itm/131094766586?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649


The full manual is here...
http://www.mppsolar.com/manual/PIP-MS 1-5K manual.pdf


and your loads.
The load will vary from zilch to around 2500w on the rare occasion. Washing machne, microwave, A/C, hair drier, electric drill and other usual bits. Fridge & stove on gas but fridge can be 240V if I want/need.

You mentioned a boat.
Yes, a houseboat to be specific... sometimes on shore power, sometimes on solar only & if needed on genny input.
Do you have solar charging?
Yes, solar at present consists of 10 X 24V nom X 190w panels. 5 strings of 2 to make 48V nom.

What are the loads?
See above.
Background information will all help us understand what you're aiming to do.

How are you charging the battery? Do you have a manufacturer and part number for the charger? What algorithm does it use?

How will it respond to having its mains supply cut off and restored while it's part-way through the charging process?
The Combo unit can be set in several modes. It CAN be set similar to a normal UPS .... switch auto to battery on mains fail........ etc etc

What is the charge current?
Charge current can be set from 10A right up to 60A. (combination of solar & grid etc) I have it limited to 20A as the couple of cells tend to 'run away' if I belt the charge in. Max of solar I've seen was 21A approx.
What is the maximum discharge current?
I try not to turn too many things on at the same time!!!
At present I'm keeping things under 'control' by connecting/turning on two powerful 12V spotlights to cells running high.
Usually cells 7 & 8 as well as cells 15 & 16 so I connect the lights across 2 cells at the same time...... basically 7.5V connected to 12V lights. I leave the charger on at this time & it appears to eventually bring things back to 'level' but ofcourse voltage does 'bounce back' a little on removing the lights!!
 

BruceS

Jun 25, 2014
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I watched a YouTube item on limiting relay contact 'erosion' & have a few questions.
Please do NOT wander onto EMF on the signal side ....
In emergency low voltage shutdown (which may never happen?) I wish to cut the current from battery to the inverter.
48V nom & anything up to 70-80A -- unlikely at time of dis-connection.
Any suggestions if it's worth having a capacitor & if so, around what size?
 
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