# Batteries and New Battery Technologies

Discussion in 'Boat Design' started by brian eiland, Mar 28, 2008.

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### portacruiseSenior Member

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### portacruiseSenior Member

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### sigurdPompuous Pangolin

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### BertKuSenior Member

Hi Porta,
I wonder how they did the BMS. Here a glimse on the project I am working on. I am able to use 4 x "AA" refer right under in the photo(thanks Sigurd for the tip, but nobody can tell me about what the correct way is on connecting batteries parallel, I assume with diodes or Power Mosfets)

The system is very flexible:
Any battery, NiMH, NiCad, LifePO4, 4 x "AA"'s Type D etc.

Any voltage, refer the large pipe, 48Volt, middle size pipe, 24 Volt or the prototype for 12 Volt.

I was not able to finish it off today, as suddenly my wife was asked to fly to Portugal for a conference and we had to rush around for all sorts of issues.

I love to get some inputs on the truth table for the BMS, I like to design. It is based on the law of an Auditor, anything out of the ordinary must be investigated or action taken.

There are two meassuring points, A at + of the 40 batteries in series and B at the plus of the 30th point. i.e. 10 batteries and 30 batteries.

for NiMH,
When we switch the BMS on,

We first place all voltages of all points A and B in the memory of the microprocessor.

Then the next step is to calculate the average of those voltages, provided they are within a certain tolerance of each other.(still to be determend)

The next step is to compare a cell with a string of 40 batteries with those averages.

if A >= 40 x 1.76 Volt , pull emergency plug on the charger
if A >= 40 x 1.45 Volt, stop charging that cell
if A <= 40 x 1.0 Volt, start charging that cell (0.1 C)
if B = outside 1 Volt, either plus or minus ring buzzer or flash LED
if B= 0 ring buzzer or flash LED

1) What did I forget ?.
2) Could I also do this method for a BMS system for LiFePO4 type "D" batteries. (with different Voltage values) What is your feelings about this ?.

Bert

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### capt vimesSenior Member

i am actually not quite sure what your "BMS" stands for but i can garantee you that it will not work for LiFePO4 batteries...

Li batteries REQUIRE one charge/discharge controller per CELL... if you place a couple of them in tubes and only one controller at the outlets you might probably overcharge single cells because they do not charge at the exactly same rate and you know what that means with Li batteries - don't you? ;-)

i have unfortunately no clue how NiMH batteries behave or have to be treated when charged...

6. ### Guest625101138Previous Member

The LiPo charger I use monitors each cell and has the ability to balance the cells minutely however all cells are charged in series not individually.

There is a separate plug that has small wires to each cell for the monitoring and balancing.

My little battery already has one cell with a slight discrepancy to the others. I have not tried a complete discharge yet.

I believe some of the lithium chemistry is less susceptible to imbalance but from what I have seen so far, all are using individual cell monitoring of some sort. I believe if a cell is completely discharged it will die.

Rick W

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### capt vimesSenior Member

sure you can put all the controllers into one place or monitor each cell with a central device put nevertheless - it is a requirement that each single cell needs monitoring and control during charging and discharging...

charging: overcharging Li cells might cause a rather explosive reaction ;-)
discharging: you are right - a completely discharge will destroy the cell

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### CDKretired engineer

Capt vimes, you are ill informed.
My 12V torch with 4 Lithium cells is charged with an ordinary 2-wire adapter. A tiny thermostat interrupts the current in case the battery gets overheated.
Monitoring each cell is only needed if you are in a hurry and charge the cells with max. allowed current.

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### capt vimesSenior Member

i dont't think that you plug in your torch and charge it with 220 V@50 Hz AC.... ;-)
so your "ordinary 2-wire adapter" might actually be a proper speced AC-DC converter charging the cells with the right amount of amps for a 1C or even only 0.1C charge having a thermostat for security reasons...

for sure this works but i doubt that you ever can charge your cells above 80%... which is of course not so much of a problem with a torch...

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### cameron.d.mmJunior Member

For those you might be wondering, BMS is used as a short form of Battery Management System. Generally all LifePO4 systems I've seen require some kind of balancing - but these have been large capacity, high charge rate, heavy usage applications (traction packs for automotive use). My impression is that there are a variety of ways the balancing can be achieved, from the simple and inefficient to the complex and expensive (but more intelligent).

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### BertKuSenior Member

Hi Capt Vimes,

Carefull, CDK is an electronic engineer and he certainly can teach you a thing or two.

I also was so foolish to think that all Lithium batteries will explode. However LiFePO4 are safe and can handle a couple of knocks, we yachtsman can do sometimes. Well in anyway I do sometimes something stupid.
Bert

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### BertKuSenior Member

Thank you Cameron.d.mm, I agree with you.

Folks I did it. I indeed found a complete simple way in making a multiple battery casing from standard pipes. I had a big problem in finding a way in putting pressure onto the top battery. The bottom battery has the weight from the other 9 batteries on top, but the top one was my problem. Here the solution.

A rubber (foot, feet ? end) which fits the 40 mm pipe.
1) At the bottom an alluminium link to the next cell
2) then the 2 - 10 batteries.
3) then the pcb plate with yellow copper screw + nut
4) under which the cable slug is mounted with flexible core
5) then a 1 cm inner tube again
6) then the rubber feet firmly pressed into the coupling.
7) final result, the rubber is clamped and put pressure onto the contact and also seals the top.

I am not planning to charge at 1C, but at 0.1 C and also the LiFePO4 should be able to be controlled in the previous proposed methode. But time will learn whether this is the case.

Bert

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### BertKuSenior Member

Sorry, I forgot to attach the 2nd photo.

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### JonathanColeimagineer

If you charge a bunch of batteries in series, how do you use a BMS simultaneously equalize one cell without charging the others? Do you use a sensor at each cell with the capability of providing a measured over-voltage with a separate wire providing the feed? How do you prevent this extra charge from feeding through to the other cells?

15. ### Guest625101138Previous Member

Jon
Attached picture shows a 4S lipoly battery. Note that it has large red and black wires for the main current but also note it has a small plug with 4 small black wires. Each of these wires is connected to the cell interconnectors so each cell can be monitored individually.

The charger I have is able to charge up to 7S battery. It is smart enough to detect the type of chemistry and it sets the charge profile accordingly. The different chemistries have different cell voltage. My LiPo cells are charged to 4.2V.

The second image shows a more sophisticated BMS with monitors/controllers attached to each cell. I expect these have some ability to balance the cells.

I have not gone into how the detectors in the little chargers work but they are certainly capable of measuring and displaying the individual cell voltage. They may use opto couplers to reduce the voltage in the control circuit.

If you look at the big stacks that are used in the electric dragsters most have LEDs for each cell. I am not sure if these simply monitor or are able to provide some ballancing.

Rick W

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