Charging a Li-ion Battery of 25.9 V and 5.5Ah

[Bob Parihar]

I'd be very surprised if that were the case.
Good luck with all your cycling. Should keep you fit .

I have read the specifications of battery.. It says it needs 29v 2.75amps for 5 hours to charge...our alternator can give only 28.4 v... Will it work?

 

[audioguru]

Do you have a Lithium battery charger circuit that will limit the charging current to 2.75A or less and limits the charging voltage to 7 x 4.2V= 29.4VDC?
Will your charging circuit detect that the charging current has dropped to about 2.75A/40= 69mA which happens when the battery is fully charged then disconnect the charger from the battery?

I think your alternator produces different voltages and currents depending on the speed it rotates. Do you have a full wave bridge rectifier and filter capacitor that will charge to (28.4VAC x 1.414) - 2V= 38.2VDC or more?
Will your charger circuit accept an input of 38.2VDC or more?

 

[Bob Parihar]

Do you have a Lithium battery charger circuit that will limit the charging current to 2.75A or less and limits the charging voltage to 7 x 4.2V= 29.4VDC?
Will your charging circuit detect that the charging current has dropped to about 2.75A/40= 69mA which happens when the battery is fully charged then disconnect the charger from the battery?

I think your alternator produces different voltages and currents depending on the speed it rotates. Do you have a full wave bridge rectifier and filter capacitor that will charge to (28.4VAC x 1.414) - 2V= 38.2VDC or more?
Will your charger circuit accept an input of 38.2VDC or more?

The rectifier can be fixed with the alternator.. Plus the electrician has told me that there is a ic circuit attached with the alternator which will automatically detect the battey type and gives a constant current constant voltage condition according to the battery needed..
Iam confused that how you calculated 38.2V DC from the alternator

 

[audioguru]

An alternator produces AC (Alternating Current), not DC. It might have a rectifier and it might be designed to charge an old fashioned lead-acid battery. Then it might set a Li-ion battery on fire.
The voltage of an alternator swings up to a positive peak then swings down passing 0V then continues to a negative peak then passes through 0V to swing to the positive peak again, over and over.
Its RMS voltage is the rating of its sinewave but its peak voltage is the root of 2 times higher (x 1.414 times higher than the RMS voltage).
A fullwave rectifier charges a filter capacitor to almost the peak voltage.

 

[Blueteeth]

I'll stick my oar in, even though its been said

Charging batteries from a source that varies in terms of 'energy output' (both current and voltage) can be difficult. The obvious method would be to only charge your batteries when the energy provided is above a certain threshold - but this can throw away significant amount of energy if its just below this threshold. You will have to, at the very least, separate it into two sections - power conditioning from a generator/alternator, and the battery charging circuit, which can monitor the battery voltage, current, temperature and handle cell balancing.

Lithium Polymer batteries are convenient for intermittent charging, that is to say providing you have an effective charging circuit, you can charge it for a minute, or several hours without the battery having any 'memory'. Laptop batteries do have a memory in a way, but this isn't the battery itself, rather the 'coulomb counter' circuits used to indicate charge level, and battery health.

I would rig your alternator to a treadmill, or a exercise bike, and run some basic tests. A rectifier of sufficient current capacity, and a large smoothing capacitor so you get 'rough DC'. Along with a load such as a light bulb, rated at a higher voltage than you believe the alternator to run at. With two multimeters you can measure voltage and current of the load. This doesn't have to be accurate at all, but it will give you an idea of how much the voltage varies, and what currents it provides under loads depending on how fast you cycle.

Very basic, but that will give you a range of voltages, and respective currents to work with. You can then use this information to design a switching converter to step up/down voltage, or provide a constant current for charging your batteries. Also, the 'constant current' charge of a lithium battery can vary, so the more energy provided the faster it can charge, but also continue to charge at a slower rate when the energy input is low. This will greatly affect charge times, and you may need a circuit to keep track of current and voltage to give you an indication of how much energy has gone into the battery (and how much energy has been drawn from it).

A 7 cell Lithium charger can be complicated enough even with a known stable power source, with a variable one it only gets worse

Your project will be complicated, and probably not that efficient, but breaking it down into parts, and testing each one will help.

 

[Bob Parihar]

I'll stick my oar in, even though its been said

Charging batteries from a source that varies in terms of 'energy output' (both current and voltage) can be difficult. The obvious method would be to only charge your batteries when the energy provided is above a certain threshold - but this can throw away significant amount of energy if its just below this threshold. You will have to, at the very least, separate it into two sections - power conditioning from a generator/alternator, and the battery charging circuit, which can monitor the battery voltage, current, temperature and handle cell balancing.

Lithium Polymer batteries are convenient for intermittent charging, that is to say providing you have an effective charging circuit, you can charge it for a minute, or several hours without the battery having any 'memory'. Laptop batteries do have a memory in a way, but this isn't the battery itself, rather the 'coulomb counter' circuits used to indicate charge level, and battery health.

I would rig your alternator to a treadmill, or a exercise bike, and run some basic tests. A rectifier of sufficient current capacity, and a large smoothing capacitor so you get 'rough DC'. Along with a load such as a light bulb, rated at a higher voltage than you believe the alternator to run at. With two multimeters you can measure voltage and current of the load. This doesn't have to be accurate at all, but it will give you an idea of how much the voltage varies, and what currents it provides under loads depending on how fast you cycle.

Very basic, but that will give you a range of voltages, and respective currents to work with. You can then use this information to design a switching converter to step up/down voltage, or provide a constant current for charging your batteries. Also, the 'constant current' charge of a lithium battery can vary, so the more energy provided the faster it can charge, but also continue to charge at a slower rate when the energy input is low. This will greatly affect charge times, and you may need a circuit to keep track of current and voltage to give you an indication of how much energy has gone into the battery (and how much energy has been drawn from it).

A 7 cell Lithium charger can be complicated enough even with a known stable power source, with a variable one it only gets worse

Your project will be complicated, and probably not that efficient, but breaking it down into parts, and testing each one will help.

Sir thanks for your time and help..
I was looking at the alternator component and i find this chart showing different parts.. Iam using the same alternator as described in the diagram attached..
Few questions...
Is it the same bridge rectifier as u were talking about..
And there is a ic regulator.. Is'nt this is supposed to give a constant voltage..
And the electrician was saying that this regultor IC could detect automatically what type of battery is attached to it.. And charge according to it.. But i have dought on what he is saying..

 

[audioguru]

The alternator is made for a car or motorcycle to charge an old fashioned very large lead-acid battery. Simply look in Google for Denso Alternator to see. DO NOT connect a Li-Ion battery to it.

 

[Blueteeth]

If the alternator is pseudo-regulated, limiting its upper voltage to a known value, you may be able to use this as a power supply for your charger. Please note, as audioguru said above you cannot connect this 'directly' to a Lithium battery.

There are *many* IC's for battery charging, that can vary from simple 8-pin linear regulators for single cells, to 64 pin entire battery management systems that can work with a wide input voltage range. I would advise you use a chip specifically designed for charging lithium batteries of the number of cells you have (7 if I read the above correctly). Almost all of these are surface mount components, would require a custom PCB, but they can also handle the cell balancing, and perhaps form a switching power supply so you can use a lower voltage as your input.

as I said before a 7-cell charger, even with a simple 35V power supply, is a complicated project. Not just to ensure long battery life and good charge times, but also so it doesn't permanently ruin the battery cells. If you can find an 'off the shelve' charger that can be powered by DC, say 48V, then I would use that, along with a boost power supply to step up your alternators voltage from ~15V to 48V. It will be very inefficient, but would probably be the fastest way to actually 'build' the project.

 

[ChrisP58]

Thanks for informing sir...
I will use some kind of gear mechanism which will generate more rpm as compared to normal..
....

The power will be the same regardless of the RPM.

Edit for clarification:
The horsepower delivered to the alternator from your pedals will not be increased by any kind of gear mechanism. Changing the RPM may cause the alternator to work better, or deliver a voltage better suited to the load, but the effective power is not increased.

 

[audioguru]

If you disconnect the voltage regulator in your alternator then it will produce more power at higher RPMs. But the battery is Lithium, not lead so the extra power will simply make it explode and burn sooner.
Lithium is a metal that burns, like magnesium and titanium. A lithium battery must be charged properly with a balancing circuit that measures and charges each cell separately, even when they are all in series.