discharge battery

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tom1
Posts: 1
Joined: Mon Nov 11, 2013 8:53 pm

discharge battery

Postby tom1 » Wed Nov 13, 2013 7:57 am

hello !!

i have several qustions about battery discharge:

1) if the battery is 5000mah and have a discharge rate of 3c then the corrent
is 15 A, but what happen if the motor consumed only 10 A current ? Is the engine overheats ?, and how this affect the battery ?

2) when i choosing a bms (battery managment) for my battery pack, what should be the bms current in the case described above, acooerding to the motor consumption or the battery ?

3) when i connect several cells in series, and each cell have a discharge rate of 3c, is the pack also have only 3c discharge rate ?

thank you all
tomer

Grumpy-b
Posts: 991
Joined: Sat Dec 01, 2007 12:06 pm

Re: discharge battery

Postby Grumpy-b » Wed Nov 13, 2013 7:42 pm

C is a reference to the stored capacity in Amp hours. Different cells can discharge at different rates. So if you have a 5ah cell that could potentially deliver 3c then it could deliver 15amp for a shorter time , ie 20 minutes. Often the higher C discharge rate can only be delivered for a short period of time.
If the motor takes 10 amp then the battery may last for 30mins. If its a 10amp motor it cant take more and so would only discharge the cell at a 2c rate.
The C discharge rating is what it is capable of delivering, and as I commented this is often time limited.
Charging is also different and may be at a 0.5c or less rate, so at 0.5c or 2.5amps the battery would take two hours to charge.

The BMS (If you wish to have one) needs to match the cells capacity, and potential charge rate. So you wouldnt have a BMS capable of working with a 160ah cell to control a 5ah cell.

When in series just consider the battery to be of a higher voltage, the AH wont change. If you connect in Parallel the voltage stays the same but the AH will increase.

Grumpy-b

jumpjack
Posts: 10
Joined: Wed Nov 20, 2013 12:16 pm

Re: discharge battery

Postby jumpjack » Thu Nov 21, 2013 3:18 pm

tom1 wrote:hello !!

i have several qustions about battery discharge:

1) if the battery is 5000mah and have a discharge rate of 3c then the corrent
is 15 A, but what happen if the motor consumed only 10 A current ? Is the engine overheats ?, and how this affect the battery ?

No, the motore "decide" how much current it picks from battery, not the opposite.
You can connect a 1000 Ah battery to a 10A motor, and the motor will just pick 10A for 100 hours.

2) when i choosing a bms (battery managment) for my battery pack, what should be the bms current in the case described above, acooerding to the motor consumption or the battery ?

The BMS must allow the maximum current required by motor to flow out from battery; hence you'll need a 10A BMS.
3) when i connect several cells in series, and each cell have a discharge rate of 3c, is the pack also have only 3c discharge rate ?

yes

Grumpy-b
Posts: 991
Joined: Sat Dec 01, 2007 12:06 pm

Re: discharge battery

Postby Grumpy-b » Thu Nov 21, 2013 6:32 pm

The BMS is about the battery management or monitoring, not motor power usage , and so should match the battery. It doesnt matter what the power being taken out is, if the battery cant cope it will fall out of its parameters and the BMS react. If you have a 100ah cell you need a BMS that can cope with that battery and it potential level of discharge. If you have a 10ah cell its pointless having a 1ah bms or a 100ah bms. Some do measure current and so this matching of the cell to the BMS is important, some merely monitor voltage and so are less critical.

Grumpy-b

jumpjack
Posts: 10
Joined: Wed Nov 20, 2013 12:16 pm

Re: discharge battery

Postby jumpjack » Thu Nov 21, 2013 7:50 pm

Grumpy-b wrote: If you have a 10ah cell its pointless having a 1ah bms or a 100ah bms. Some do measure current and so this matching of the cell to the BMS is important, some merely monitor voltage and so are less critical.


BMS works on A, not Ah.

The design sequence is:
- chose the power of the motor (e.g. 2000W)
- choose the voltage of the motor (e.g. 48V)
- calculate the current from power and voltage (2000/48 =~ 40 A)
- choose a BMS capable of extracting such a current from a battery (40A)
- choose a battery which gets damaged as low as possible by such a current (batteries don't like being discharged...) (80 Ah)

Additionally, you must take into account peak power of the motor, i.e. power it can require for a few seconds, which could be 2 times the rated power, e.g. 4000 W --> 80 A.
BMS have indeed a "continuous rating" and "peak rating".

Grumpy-b
Posts: 991
Joined: Sat Dec 01, 2007 12:06 pm

Re: discharge battery

Postby Grumpy-b » Thu Nov 21, 2013 9:13 pm

The BMS is about Batteries and either monitoring them, their voltage and the amount of energy they have been given during the charge and how much is taken out during usage. Or they are a Battery management system, attempting to control the way they charge not allowing over charge, possibly balancing the final charged voltage and bleeding the highest down. Its not about Motors or what ever the load is.
The AH of the cell is also a guide to the capabilities of its discharge, but all cells are not equal. SO you then need to consider your whole system. If you have a motor that has a max current of say 100amps, you should have a cell capable of that . It could be that you have a cell that can manage 1c discharge so you would have a 100ah (Generally the Ah is expressed at 1c) cell, but if you dont need a long life between charges, you could consider a 20ah cell capable of 5c discharge.
Each of these will require a different BMS, ie the batteries are different one needing to monitor and charge/ balance a 100ah cell, the other doing the same for a 20ah cell.

So you need to understand what you are trying to achieve, what the capabilities of the cells you wish to use ae, and so what the appropriate size is for your purpose and what BMS will suit those cells. Also consider the charger.

Many BMS dont monitor the current, so frankly dont really care what you discharge, is
all they are trying to do is prevent you from over or under voltageing the cells. If they actively balance the cells, then they need to match the charge capabilities of the charger and the cell. If you are charging at 50 amps a bleed current of a few ma, will be point less. Hence the whole st up needs to be matched. Many of the on cell systems (Calb Sino poly etc cells) fit the specific AH cell that you have chosen, and are matched to the cell characteristics (Charge voltage and final voltage, as well as low volt cut off) some have a single current shunt or other sensor that will monitor the discharge and charge at a pack level, and use that as well.

Others will choose to use no BMS,
I work with vehicles on a daily basis with and with out. With a monitoring and balancing system, a poor cell will stop the charge dead as that cell goes high, leaving the rest of the pack under charged. But the same thing can happen if the cells are good but really unbalanced with some holding say 120ah and some holding 150ah, It wont matter how the voltage gets bled down to balance the pack, since as a voltage in the main body of the charge/ discharge curve the cells will hold a high amount of overall ah in a very small voltage range. Thats the point of all lithium cells , their high degree of flatness of discharge / charge curve.
In all builds I always ensure that the cells are bottom balanced. With Thunderskys Calb etc starting at about 2.7 resting voltage, then charging from there as a pack. If the cells are good they will all rise quite closely , and if of an equal AH they will hit high voltage together. Or yoiu can have a slightly higher Ah capability on the cell, say go for a 180ah rather than 150ah or an extra cell in the pack, and just not attempt to get to the absolute high voltage, / High ah ie dont take a Thundersky to 3.8 but to 3.5v so that way you wont over charge.
I love the use of a BMS when I can extract the individual cell data, it helps to find out the problems, and show which cells have problems during charge and discharge. You cant tell this from static voltages.

WHat ever you do it will be a compromise.If you cant afford good quality cells, then the best BMS in the world wont help you out, but a Good quality cell, will be worth preserving. Some Cell manufactureres require the use of a BMS but dont specify what they want it to do.
Grumpy-b

GregsGarage
Posts: 870
Joined: Tue Apr 01, 2008 5:27 pm
Location: Galashiels, Scottish Borders
Contact:

Re: discharge battery

Postby GregsGarage » Fri Nov 22, 2013 10:18 am

A bms must monitor individual cell voltages and act when any cell goes to high or low. On ebikes the usual way is to simply disconnect the pack from the charger/controller with mosfets on the bms board. Since you don't want to overload the mosfet they will implement a current monitor circuit that will also disconnect the pack from the charger/controller if the current limit is reached. A added benefit is that you now know your pack, controller and motor will never see more power than what the bms will allow. You can fit a higher current bms which would allow more current from the pack to the motor, but you may not actually see any more current. Actual current depends on the controller, motor and pack conditions such as pack voltage, motor rpm, load, etc. And if you do actually increase the power, can your cells, motor and controller handle it?

This simple ebike bms works fine and the sudden loss of power isn't a problem, you can always pedal. But when you move up to higher power vehicles, cars, motorcycles, etc., the BMS implementation typically changes. The ideal method is to have the bms directly intervene in the charger and controller to start limiting current and ultimately shut down the charger/controller to protect the pack.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191


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