Alternator Size with LiFePo4

[rgleason]

Thank you John.
1.2a came from a forum post. Dale confirmed 1a this morning

Since the Winston batteries state 100a as optimal, which is 0.5C, using those would be best, at .6C for cruising. Then when at sea I would bump it up to .7C.

Heat and continuous output

Balmar advises that difference in cold (72f) and hot (200f) output curves are about 10-15% reduction. It is available on their website.
Since my alternator temp sensor is mostly below 74c and occassionally at 77c, which is 170 degreesF, which located is near the diodes and a very good calue, however the rotor runs 30 degreeF hotter, so essentially my alternator is running at 200 grees. Many others will be hotter.

The output is dependent on many conditions: SoC, type of battery, belt slip, belt type, venitlation in the engine compartment, temperature of the compartment, etc.

With rotor core temp above 200 there will be additional loss of output.

Mitigation of heat:
- Fresh air blown on alt.
- Venting
- Serpentine belt
- Belt cowl replace with grate or remove
- Then resort to a remote rectifier which might add 20a to outputt. - Is the extra $ cables and gear worth that?

General rule is to
Size alts at least one size bigger and run belt manager at b-3 which reduces top ourput by 15% in order to have longer life under continuous load.

[rgleason]

Tanglewood, your corrections are much appreciated.

We need a new solid state way to convert mechanical energy into electrical power, or perhaps a solid state way to convert engine heat to power!

[tanglewood]

Originally Posted by evm1024

Two diodes with a nominal 0.6 volt drop each gives 1.2 volts drop across the pair. 140 amps through a 1.2 volt drop gives 168 watts of heat to dissipate.

So I may just be really rusty at this, but I'm pretty sure the calculation is with only one diode drop since only one is conducting at any given time. In a back to back config like the full wave rectifier, if one is forward biased, the other can't be, and vice versa. So I don't think there is ever a drop across both at the same time, just alternating which diode is conducting at any give time.

[evm1024]

Two conduct at one time - one to the 12 volt output and the other to the ground.

The circuit is ground to negative diode, to the first coil, then to the second coil then to the positive diode to the output.

Just ignore the diode trio in this photo...

[tanglewood]

Originally Posted by evm1024

Two conduct at one time - one to the 12 volt output and the other to the ground.

The circuit is ground to negative diode, to the first coil, then to the second coil then to the positive diode to the output.

Just ignore the diode trio in this photo...

I understand the circuit, but isn’t only one diode conducting at a time?

[evm1024]

Originally Posted by tanglewood

I understand the circuit, but isn’t only one diode conducting at a time?

One positive and one negative diode at a time - two diodes. Dies this help?

[rgleason]

Sizing Alternator to the planned Battery Bank

Example:
200ah battery Bank
x.6C charge rate for LiFePo4
120a amps needed from Alternator (minimum)

120a x 1.2 =144a needed due to operation in a hot area.

If the engine compartment is > 170degF then add 30degF for rotor temperature so the alternator temperature is 200degF. If it is higher will probably need a larger alternator.

144a x 1.2 =172a Alternator cold rating to run Belt Manager B-3 for longevity and continuous duty.

EVM1024 Thanks for your diagrams and explanation.

[evm1024]

Of course it is more complex because it is 3 phase. For the math part we just need to consider the current and voltage drop to get the total dissipation.

Lucky for us someone has made a vedio of the current flow and the steering that the positive and negative diodes do.,

See here:

https://www.youtube.com/watch?v=V6ZWYFTELQA

[tanglewood]

Originally Posted by evm1024

One positive and one negative diode at a time - two diodes. Dies this help?

It does, thanks. I was indeed rusty.

[rgleason]

That does help. You can understand why 3 phase is used too.


Several other things from Balmar regarding use of Alternators with LiFePo.
1. The BMS should interrupt the ignition wire to prevent frying the diodes, rather than the field wire. This way when the alternator is started again there will be a soft start with a 45 second period and then a ramp up.
2. Litionics Battery LiFePo BMS has good dedicated controls and relay for alternators. They have batteries in many sizes which plug in to their BMS quite simply, but they are very expensive.

[john61ct]

Could you post example pricing at different sizes?

More or less than Victron?

[witzgall]

Diodes and heat:
I have heard this before. While the rectifiers certainly give off a good deal of heat, it is well south of 50% of the total heat given off by a running alternator. The stator is far and away the biggest generator of heat in an alternator. The diodes in the rectifier are the most vulnerable to damage by heat. Consider a rectifier of a 300a alternator. The diodes are collectively dissipating a 1v drop at 300a, or a total of 300 watts.


Cooling a small case alternator:
As an example, the 6-series Balmar alternators have dual internal fans, typical of many small case alternators. They draw air from both the front and rear of the unit, and expel the air out of the sides of each fan. In small engine rooms, or those with bad circulation, you can help alot by BLOWING outside air onto the back of the alternator. By doing this you are helping to manage the heat at the diodes, and of the overall temp of the alternator.

Originally Posted by rgleason

Under "Alternator Heat Management"
"The majority of the heat from an alternator comes from the rectifier diodes, which convert the alternator's AC power to DC. Most alternators draw air from back to front. Unfortunately, the air behind the alternator in most installations is typically beside the exhaust manifold of the engine - the hottest place in the engine compartment. Alternators with internal rectifiers can get so hot, you could fry an egg on them, and many can get hot enough to melt the epoxy insulation on the stator wiring when operated at fullfield current"


"

[deluxe68]

Originally Posted by witzgall

Diodes and heat:
I have heard this before. While the rectifiers certainly give off a good deal of heat, it is well south of 50% of the total heat given off by a running alternator. The stator is far and away the biggest generator of heat in an alternator. The diodes in the rectifier are the most vulnerable to damage by heat. Consider a rectifier of a 300a alternator. The diodes are collectively dissipating a 1v drop at 300a, or a total of 300 watts.


Cooling a small case alternator:
As an example, the 6-series Balmar alternators have dual internal fans, typical of many small case alternators. They draw air from both the front and rear of the unit, and expel the air out of the sides of each fan. In small engine rooms, or those with bad circulation, you can help alot by BLOWING outside air onto the back of the alternator. By doing this you are helping to manage the heat at the diodes, and of the overall temp of the alternator.

Recirculating air in the engine compartment was just discussed in another thread. The general consensus was that it is a bad idea to blow outside air into the engine compartment. I plan on cooling my alternator by running an exhaust hose to the side of the alternator. One thing about the Balmar small case alternator is that the diode case looks like it may restrict the airflow. I will take a better look at it next spring.

[rgleason]

Originally Posted by witzgall

Diodes and heat:
I have heard this before. While the rectifiers certainly give off a good deal of heat, it is well south of 50% of the total heat given off by a running alternator. The stator is far and away the biggest generator of heat in an alternator. The diodes in the rectifier are the most vulnerable to damage by heat. Consider a rectifier of a 300a alternator. The diodes are collectively dissipating a 1v drop at 300a, or a total of 300 watts.


Cooling a small case alternator:
As an example, the 6-series Balmar alternators have dual internal fans, typical of many small case alternators. They draw air from both the front and rear of the unit, and expel the air out of the sides of each fan. In small engine rooms, or those with bad circulation, you can help alot by BLOWING outside air onto the back of the alternator. By doing this you are helping to manage the heat at the diodes, and of the overall temp of the alternator.

To be clear, I did not make that statement, it was a quote. Yes from tanglewwod and evm posta below it is clear where the heat comes from.

With a small 65a alt a bilge fan blowing fresh air keeps it at 74C when I change to a much bigger alt that will go up.

[rgleason]

Which thead was discussing this please?

I spoke with Balmar yesterday about a new larger alt and output while hot and they actually suggested blowing air on the back and making a grill instead of the belt guard in front. So that is what I plan to do.