LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[T1 Terry]

we use a lithium battery to start the Hino diesel, the cell logger doesn't record a spike even at 0.2 sec sample rate, i think the fact lithium batteries are close to a super capacitor as far as their ability to accept a charge the spike never gets a chance to happen.

T1 Terry

[hellosailor]

Terry, if your cell logger is recording every 0.2 seconds, it won't show starter spikes. First of all, 0.2 seconds is 200 milliseconds. Offhand, the spikes last something like 1-5 milliseconds in total. That's simply too fast for most equipment to see it, whether it is averaging or peak-reading or whatever. If you aren't using something like an oscilloscope to watch for very short duration events--you won't see them.

And then of course a super capacitor is not part of a normal battery install--even of normal lithium types--anyway. The spike DOES HAPPEN, make no mistake about that. it may not get a chance to pass the supercapacitor, which will be a good surge/spike supressor, but that doesn't change the physics of what happens when the starter throws spikes.

[T1 Terry]

I think you miss understood my post, the lithium cells accept charge so easily because of there very low internal resistance, just like a super capacitor, they are built the same way using the same electrolyte and materials. Pulse charging they will accept over 20C, that would absorb the pulse from a stick welder, there needs to be resistance for the voltage spike to build, the resistance is so low in a lithium battery it just doesn't happen. After the christmas panic season is over I'll connect a CRO across a 24v lithium battery and weld a few lines, that should produce a spike if one is going to happen shouldn't it?

T1 Terry

[mcarling]

A better test would be to hook up an oscilloscope while starting the diesel. Record the peak voltage between the battery and your electronics. I'm nearly certain that lithium phosphate batteries will do a much better job of flattening the spike than lead acid batteries would, but I do not guarantee any particular results.

[daddle]

A capacitor is a poor choice for absorbing noise from a motor in a power circuit. A diode directly parallel to the motor is the better choice. I am not certain how one is selected. Perhaps 50V and 50A. Attach the cathode (marked terminal) to the positive terminal of the motor. Perhaps another diode parallel to the solenoid.

That said, I don't think starter motors make a significant voltage spike in a proper installation.

[T1 Terry]

Originally Posted by daddle

A capacitor is a poor choice for absorbing noise from a motor in a power circuit. A diode directly parallel to the motor is the better choice. I am not certain how one is selected. Perhaps 50V and 50A. Attach the cathode (marked terminal) to the positive terminal of the motor. Perhaps another diode parallel to the solenoid.

That said, I don't think starter motors make a significant voltage spike in a proper installation.

I think you will find that diode and more is already part of the power supply to the electronic equipment, it couldn't function as designed without a stable power supply.
Keep in mind, that radio suppressor fitted to the coil on your old points ignition car was a little capacitor, they work fine for noise suppression on DC electrical circuits

T1 Terry

[hellosailor]

Terry-
Arc welding usually throws nice spikes IIRC, yes. What's a CRO and what are you using to measure them though?

Daddle, if you take a look on the web, you'll find that one of the premium suppliers of LEDs is a company called Cree. In their engineering application papers, they specifically warn automotive customers (US, German, Japanese, pick a brand name) that if no spike protection is used, they will not warranty their pricey LEDs at all. Doesn't matter how good the installation is or how good the starter is, when the starter kicks out, power is removed but it is still spinning in a collapsing magnetic field--which makes it however briefly into a generator, and a rather effective one.

Collapsing a magnetic field is, on a somewhat different scale, how electromagnetic pulse devices (sometimes called EMP Bombs) can be made to wreak havoc. The underlying physics is still the same. When a magnetic field collapses, the energy will try to go SOMEplace, and that convenient wire coil in the starter makes a great place for it.

Diodes are problematic, like all spike protection equipment. They are cosumable devices in that application. In fact one of the Delco companies (I can never keep them straight) for a while made alternators with spike surpressors built in. And found out the hard way, the failure rate in those "better" alternators was so much higher because of the device failures, that they killed the design. This is why bascially all of your electronics are built without spike protection, and you're left to install and maintain it as separate feature. Or ignore it, as most folks do, and just replace things when they mysteriously die. "Well, we got ten years out of it."

[daddle]

So Cree LEDs are very fragile. They should be more prudent in their designs. Diodes as noise suppressors are definitely not a "consumable" item. There billions of them in exactly that service. Especially in DC motor circuits designed by electronic engineers. I well understand the physics. I wouldn't be so sure starter motors are like EMP bombs.

I would think automotive designs would be the last place to look for thoughtful engineering.

[T1 Terry]

Hi Hello Sailor

Doesn't matter how good the installation is or how good the starter is, when the starter kicks out, power is removed but it is still spinning in a collapsing magnetic field--which makes it however briefly into a generator, and a rather effective one.

The part you have missed is the solenoid that powers the electric motor is now open circuit, there is no current in the field winding (think I got the name right) so it simply becomes a spinning armature, not a generator. The only spike is from the field collapsing in the solenoid windings and the power wire isn't connect there any more either so it's only way out is through the starter windings, used in milliseconds. The surge voltage is what's been travelling down the cables suddenly stops, just like the thud or hammer in the water pipes, an accumulator or sensitive over pressure relief valve or a dump valve stops that hydraulically, a capacitor or diode or both does it electrically. Lithium batteries are very big capacitors, if they failed the starter wouldn't functioned in the first place would it?

A CRO is a Cathode Ray Oscilloscope.

T1 Terry

[hellosailor]

More likely it is better economy to design one spike protector to match the installed environment, than to make 50 LEDs (there are typically more than that in one car installation) more robust. An LED is, after all, just a diode and if diodes were all that was necessary to protect things...LEDs would never blow out either, would they?

Fragile? All electronics are fragile. Thoughtful engineering? Perhaps you've forgotten, automotive engineers are in the business of thoughfully shaving a half penny off each vehicle, and saving millions of dollars toward the profit line every time they do that. They're not all trainees from Fiat and Lada.

The engineers who designed your boat were well aware that it could be sunk and lost. Does that make their design bad? Perhaps not. Perhaps it just means "some adult supervision may be required" for the intended use.

Terry-
Interesting, CRO doesn't translate into Colonial. Here they are all "scopes" (one syllable instead of three) for short, regardless of the tube, analog or digital, whatever.

[mcarling]

Originally Posted by Hyprdrv

And that is why I took the Genset off the House bank and dedicated a battery specificly to it. Now there is no motors on the house bank.

You have two diesel engines and one diesel genset each with its own battery? If the pumps are not on the house bank, how do you power them? Do the pumps have their own bank?

[Hyprdrv]

Becouse my motors for Propulsion are electric and run off a seperate bank of 12 batteries. Pumps I assume you mean Bildge pumps? They are on the house bank but if I have water running into them I have more issues than my Electronics blinking off and on.
Steve in Solomons MD

[T1 Terry]

Originally Posted by hellosailor

Terry-
Interesting, CRO doesn't translate into Colonial. Here they are all "scopes" (one syllable instead of three) for short, regardless of the tube, analog or digital, whatever.

:lol: just known as a cro(w) around these parts, even the digital ones, a "scope" is a transformer based temp controller soldering iron, a brand name thing I guess just carried over, funny how an abbreviation just sticks some times.

T1 Terry

[mcarling]

Originally Posted by Hyprdrv

Becouse my motors for Propulsion are electric and run off a seperate bank of 12 batteries. Pumps I assume you mean Bildge pumps? They are on the house bank but if I have water running into them I have more issues than my Electronics blinking off and on.

I mean all electric pumps, whether for raw water, fresh water, bilge water, grey water, fuel, or anything else. Assuming your house battery bank and your genset starter battery bank are the same voltage, I would put all the pumps on the genset starter bank rather than the house bank in order to minimize the risk of voltage spikes damaging sensitive electronics. I'm assuming your propulsion bank is a higher voltage.

[mcarling]

Originally Posted by T1 Terry

just known as a cro(w) around these parts, even the digital ones, a "scope" is a transformer based temp controller soldering iron, a brand name thing I guess just carried over, funny how an abbreviation just sticks some times.

That's why I always write oscilloscope. It's unambiguous worldwide. I always seemed to get in trouble when using slang in any international forum.