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Lipo hybrid systems

Posted: May 26 2020 5:27pm
by unklegrumpknee
Thought experiments on parallel chemistry.
One idea is to use a primary battery to give the lipo low voltage protection.
I've thought of a Davies's battery.
1.1 volt constant. 4 would be 4.4 volts. A high amp diode about .75 volts. So it would "kick in" at 3.65 volts, no?
I would like to use this on off grid system.

Re: Lipo hybrid systems

Posted: May 26 2020 6:55pm
by JackFlorey
unklegrumpknee wrote:
May 26 2020 5:27pm
1.1 volt constant. 4 would be 4.4 volts. A high amp diode about .75 volts. So it would "kick in" at 3.65 volts, no?
Generally better to just parallel them. For years I used a 36V (10S lipo) in parallel with a 36V (30S NiMH) hub battery. Charge them separately, of course.
I would like to use this on off grid system.
You don't need that for an off grid system. More batteries are generally better than two types of batteries.

Re: Lipo hybrid systems

Posted: May 27 2020 2:14am
by flat tire
If you're making an off grid system you're better off just using a larger battery to handle your discharge requirements. Or, a chemistry like LTO that combines very high discharge with great safety and lifespan at the cost of weight. Paralleling different chemistries does work within the voltage overlap range but RC lipo finishes safe discharge at much higher voltage than other lithium chemistries so the hassle of managing that probably isn't worth the limited benefit.

Re: Lipo hybrid systems

Posted: May 29 2020 1:03pm
by unklegrumpknee
Just like the PV panels, getting enough to do 100% of the power is maybe twice the size of a system doing 80%.
Similar with batteries.
I was hoping to use the Davies's battery to provide low voltage protection. Also to power monitoring and control systems that should always have power.

Re: Lipo hybrid systems

Posted: May 29 2020 7:21pm
by John in CR
What's a Davies's battery? My search comes up blank.

Since you need something to protect your battery if away for an extended period, or no energy production for a long period, then you need a low voltage cutoff to protect your pack from low voltage unless you use a nickel based battery which can go to zero.

Use whatever battery is most economical over the long-term since size and weight doesn't really matter.

Such a diode would be wasting a tremendous amount of energy, so where's that heat going, and what happens when it fails?

As far as mixing battery chemistry goes, 23s of LiFePo4 parallels nicely with 20s of lipo, and the LFP helps prevent the lipo from going too low until the LFP runs out of juice too.

Re: Lipo hybrid systems

Posted: May 29 2020 7:36pm
by flat tire
If you can afford appreciable solar panels, mounts, and the rest of the system it shouldn't be much of a problem to amass the kind of lithium bank that can put out more power than your house / home workshop needs even with low C-rate cells.

LiFePO4 drops almost twice as much resting voltage to discharge a similar energy content as lipo. You obviously don't know anything about this or you would have already done it, not asking for validation of ill-conceived schemes on ES of all places. So, DON'T TRUST YOUR IDEAS. Follow someone else's setup that looks legit.

A properly sized stationary battery with decent cells will not need a "booster" to keep up with your power demands. This is a subject that has been covered to death on YouTube, in written literature, and in electronics discussion venues more competent and subject-oriented than this place.

Re: Lipo hybrid systems

Posted: May 29 2020 7:53pm
by amberwolf
John in CR wrote:
May 29 2020 7:21pm
What's a Davies's battery? My search comes up blank.
My guess is a reference to the Voltaic Pile used by:
https://en.wikipedia.org/wiki/Humphry_Davy

Re: Lipo hybrid systems

Posted: Jun 02 2020 5:12am
by unklegrumpknee
A Davys battery is a copper copper sulfate zinc zinc sulfate battery that was used to power the telegraph.
Been around a long time.
I don't want to turn off the lithium battery when it reaches LVD. I want to be able to power some very low draw equipment at all times. Security and temp monitoring.
Justin from Grin maintains that the death of most lithium batteries is caused by the draw of the BMS.
This system would eliminate that problem.
The energy loss from the diode wouldn't be very large because the draw would be small. Just slightly more than self discharge.
I have a world class education in chemical engineering but not a lot in electronics so you might say that I only have a hammer and every problem is a nail.

Re: Lipo hybrid systems

Posted: Jun 02 2020 9:07am
by john61ct
There is no such thing as "turning off" a battery.

The contactor controlled by the BMS can be wired to leave everything powered except the motor.

Or just sound an alarm, visual or auditory.

Re: Lipo hybrid systems

Posted: Jun 03 2020 5:40am
by dogman dan
If this stuff is so low draw, a moderate size separate battery without any lvc attached could power it. Something like your older, pretty wore out lead.

Re: Lipo hybrid systems

Posted: Jun 08 2020 9:56am
by unklegrumpknee
dogman dan wrote:
Jun 03 2020 5:40am
If this stuff is so low draw, a moderate size separate battery without any lvc attached could power it. Something like your older, pretty wore out lead.
I considered that myself.
But then I thought that I could run electronic and lvc protect at same time. If I had stored solar electronics run from that. Davys battery not rechargeable so diode is mandatory. They stay at 1.1 volt till the very end.