NiMH vs. AGM Cells

[solarnoobie]

Is the disadvantage of NiMH cells as battery banks solely because of cost?

I like the fact that you can discharge NiMH cells 100%!

AND, you don't need to buys as much Amp-Hr as AGM, since with AGM cells, you can only discharge ~ 20%.

So to use 1kw of energy from a battery bank, you will only need 1kw of NiMH cells, but 5kw of AGM cells.

[Mike90250]

I think AGM has recharge efficency of about 95%, Flooded is 80% and NiMh is about 50%, so you would need a lot more solar PV to recharge them with.

[Sunking]

Well size it up like this. For value comparison determine the cost per Watt-Hour capacity.

Flooded Lead Acid = $0.13/wh
AGM = $0.20/wh
NiMH = $1.40/wh
LifePo4 = $3.00/wh

Pretty much tells you what you need to know. So if you need 1 Kwh per day a FLA battery will cost you 5 x 1000 x .13/wh = $650

For AGM = 5 x 1000 x .20wh = $1000

NiMh = 1.2 x 1000 x $1.40 = $1680

LiFePO4 = 1.2 x 1000 x $3 = $3600

Pick your pain level. Note if you were to pick NiMH, you would need a custom made charge controller because NiMH batteries use a very special charging algorithm, and it is not compatible with solar power.

[Sunking]

One more comment. There is a bit of a flaw in your logic. Let's say Lithium and/or NiMH cost the same as FLA on a watt hour basis, and you sized your battery for just 1 day. What are you going to day when you have a couple of cloudy days? Sit in the dark?

[Sunking]

Hey I was just on a Electric Vehicle forum I frequent a lot and found some really great news. LiFeP04 battery prices have fallen to $0.44/wh down from $3/wh.

At this price point, 80% DOD capabilities of LiFeP04, and 10 year life span makes them a perfect choice for an off-grid application.

As an example let's say you need 1 Kwh per day. Run the numbers for both FLA and LifeP04. For FLA we need at minimum 2.5 days reserve capacity to carry use through cloudy days, and multiply that by 2 to stay withing 50% DOD of FLA. So using real numbers the FLA capacity needs to be taking into account charge/discharge efficiency = 1 Kwh x 1.5 x 5 = 7.5 Kwh reserve capacity. So at $.13/wh x 7500 wh = $975 for a 5 year battery, and $2000 for 10 years.

OK for a LiFeP04 system of equal 2-1/2 day reserve capacity and taking into account charge efficiency = 1 Kwh x 2.5 x 1.11 = 2.8 Kwh reserve capacity. so $.44 wh x 2800 = $1232 for a 10 year battery. That now makes Lithium competitive with flooded Lead Acid Batteries.

However that is not really the great news, just a perk. The really good news with a cost of $.44/wh now makes it possible for an electric vehicle to replace ICE (Internal Combustion Engine) You can now make a midsized EV with 300 mile range, 20 minute recharge time, with all the bells and whistles of an ICE car at a competitive price of the equal ICE car. Th eEV would be about 25% higher in price but can be justified with fuel and maintenance cost savings. with a battery to wheel efficiency of 300 wh/mile and electric cost of 13 cents per Kwh comes out to a fuel cost of $0.13 / 3.33.mile / Kwh = 3.9 cents per mile electric cost. Compare that to a ICE vehicle that gets 25 MPG and a fuel cost of $3 per gallon and you get $3/25/MPG = $0.12 /mile. The EV fuel cost is 1/3 that of an ICE car.

Think about it right now we have the technology to eliminate all foreign import petroleum for light vehicle transportation. All you need is to rebuild the electrical grid to handle that capacity using uranium as fuel or even coal or NG. We have plenty of all 3 right here in the USA.

There is only one little flaw with LiFeP04 batteries is the Chinese are the only country that can make and sell LiFeP04 batteries for $0.44/wh. Best a USA company can do is $3/wh due to labor, regulations, and tax cost. So that means China would become the worlds dominant superpower and economic giant. Well heck they already are, they bought the USA from Obama and Bush. As the ole saying goes; money goes where it is treated well and with respect.

[solarnoobie]

Anyone familiar with BMS (Battery Management Systems) for LiFePO4?

BMSs are like charge controllers, right?

I see them for electronic vehicals / scooters online, but no one seemed to have implemented them for solar panels.

Can you hook up inverters directly to the LiFePO4 batteries? The voltages seem to be off. Most inverters are 12V/24V, but LiFePO4's typical voltages do not have combos that fall into these two distinct voltages.

Thanks

[Sunking]

Anyone familiar with BMS (Battery Management Systems) for LiFePO4?

BMSs are like charge controllers, right?

I have some experience and interest in BMS. To say they are like a charge controllers is a bit of an understatement. Yes they do monitor and regulate the charge process, but they control the charge to each individual cell or group of cells. They also monitor each cell voltage, cell temperatures, and various other aspects of the batteries. Yes they are like charge controllers to some degree, but way more complex.

I see them for electronic vehicals / scooters online, but no one seemed to have implemented them for solar panels.

That is because there is no demand for them or people willing to pay for it. A EV is a completely different animal. They use up to 800 to 10,000 individual battery cells about the size of a AA battery

Can you hook up inverters directly to the LiFePO4 batteries? The voltages seem to be off. Most inverters are 12V/24V, but LiFePO4's typical voltages do not have combos that fall into these two distinct voltages.

Yes they can be used. A 12 volt inverter is made to work from the low end of 10.5 volts up to 16 volts. Double that figure for 24 volt systems. Lithium cell voltages varies a little bit depending on the exact chemistry of 3.2 to 3.6 volts, so 4 in series will give you 12.8 to 15.2 volts.

[Mike90250]

Lets see what these links are good for:

batteries & other resources you may not have handy (yet). Enjoy

http://www.fieldlines.com/story/2009/8/26/71752/6793

http://www.diyelectriccar.com/forums/index.php

[Adonyx]

Is the disadvantage of NiMH cells as battery banks solely because of cost?

I like the fact that you can discharge NiMH cells 100%!

AND, you don't need to buys as much Amp-Hr as AGM, since with AGM cells, you can only discharge ~ 20%.

So to use 1kw of energy from a battery bank, you will only need 1kw of NiMH cells, but 5kw of AGM cells.

Yes. The primary disadvantage of NiMH battery banks is cost. The secondary concern is points of failure. Each NiMH cell is 1.2v nominal, therefore using this chemistry will be tied for first place with NiCd to yield the most potential points of failure of the available and acceptable battery chemistries to choose from. The good news however is that NiMH is probably the most reliable, safest, sturdiest chemistry available of your options for high density cells, so as long as your connections are good and you use a reputable manufacturer, failures are of little concern. Still very pricey to use the good stuff.

[Adonyx]

I think AGM has recharge efficency of about 95%, Flooded is 80% and NiMh is about 50%, so you would need a lot more solar PV to recharge them with.

Misinterpretation on the 50%.
When charging a NiMH cell, on average the energy required will be C+50%, that's capacity plus 50% of capacity or 150% required to do 100% of the charge, so the "average" efficiency for charging NiMH is actually 66.66666etc%

[Adonyx]

Well size it up like this. For value comparison determine the cost per Watt-Hour capacity.

Flooded Lead Acid = $0.13/wh
AGM = $0.20/wh
NiMH = $1.40/wh
LifePo4 = $3.00/wh

Pretty much tells you what you need to know. So if you need 1 Kwh per day a FLA battery will cost you 5 x 1000 x .13/wh = $650

For AGM = 5 x 1000 x .20wh = $1000

NiMh = 1.2 x 1000 x $1.40 = $1680

LiFePO4 = 1.2 x 1000 x $3 = $3600

Pick your pain level. Note if you were to pick NiMH, you would need a custom made charge controller because NiMH batteries use a very special charging algorithm, and it is not compatible with solar power.

NiMH is $0.54 per WH retail at the distributor I use. And the price they give me is considerably lower. As for the charging algorithm, that's not true at all. There's a lot of fostered hocus pocus in the battery industry because it aids in keeping costs high if you can convince people the technology is more complicated than it really is. The reality is actually much more boring.

There are two main ways that NiMH "smart chargers" determine a NiMH battery is fully charged. One is the "negative delta v" method. When a NiMH cell reaches peak voltage and capacity there is a sudden voltage drop of about 10mv per cell. The smart charger detects this sudden slight drop and terminates the charge. An example of a common basic "smart charger" is one that just looks like an AC adapter with red and black wires coming out of it and a label that will read something like "For NiMH battery packs 12v-16.8v". Now the sales rep will tell you that it has a computerized detection circuit that determines how many cells are in the battery pack and it will adjust to the proper voltage automatically. That's a load of crap actually. In reality this particular charger is just a 25v 1.8 amp ac adapter, because at low amperage 25v is adequate to charge batteries in the 12v-16.8v range. The only thing smart about it is the added chip to detect the sudden 10mv voltage drop at the end.

The other popular method of charging NiMH cells is a thermal switch and a relay. Much simpler for DIY'ers to make. A lot of tool battery chargers use this method without issue. When a NiMH cell is fully charged at a decent rate, lets say 0.5c, that means a rate of half capacity or 2.5 amps for a 5ah battery etc, it will start to get hot. When it gets hot it will trigger the thermal switch you've taped to one of the cells in your battery pack/bank. When the thermal switch is triggered it sends voltage to a relay that shuts off the flow of power from your charging source. Easy peasy. How hot should it be to trigger the switch? A good rule of thumb is if it's too hot to comfortably handle, or about 130-135f. NiMH is a hardy chemistry and quality cells are safe to operate up to 140f.

So yes, it is highly compatible with solar using either of these methods, or neither of them if you select you power match your battery bank adequately with your solar panel. I can say this because I build battery packs, I work with them daily, and I use a solar charged battery bank to light my work space every night, and for mine I use no special circuitry what so ever. It is wired directly to the panel. All I did was power match the battery bank to the panel for capacity and daily output, and use a 10C high discharge nimh cell.

[Adonyx]

One more comment. There is a bit of a flaw in your logic. Let's say Lithium and/or NiMH cost the same as FLA on a watt hour basis, and you sized your battery for just 1 day. What are you going to day when you have a couple of cloudy days? Sit in the dark?

My method, which has been working for quite some time now, is to go with a monocrystalline panel with a peak amperage output equivalent to 1/9c of the battery bank. Works like a charm and I use it every day, cloudy or not.

[russ]

Hi Adonyx - Welcome to Solar Panel Talk!

[Adonyx]

Yes they can be used. A 12 volt inverter is made to work from the low end of 10.5 volts up to 16 volts. Double that figure for 24 volt systems. Lithium cell voltages varies a little bit depending on the exact chemistry of 3.2 to 3.6 volts, so 4 in series will give you 12.8 to 15.2 volts.

Ok, so here's the problem with using lithium with inverters...
First, the 3.6v variant of lithium cells are "primary cells", a fancy way of saying disposable. So lets rule those out.

Then that leaves us with the usual suspects of Lithium-Ion (3.7v), Standard Lithium Polymer (3.7v), High Rate Lithium Polymer (3.7v), LiFePo4 (3.2v), and most recently Nano-Phosphate Lithium Ion (3.7v).

So as we can see, typical voltage for lithium based cells that are capable of high discharge required by an inverter are 3.7v per cell.

***By the way, if you're using lithium chemistry cells for an inverter or ANY high discharge application you MUST ENSURE THAT YOU ARE GETTING A LITHIUM CELL RATED FOR 10C OR HIGHER. Mess with that at your peril, as mishandled lithium batteries are highly volatile and very dangerous. They can and will explode, vent deadly gasses, or suddenly burst into bizzarely intense flames that can not be extinguished using traditional fire extinguishers***

There are two safe and reliable lithium chemistries on the market that are abuse tolerant and durable enough not to require special safety circuitry to prevent them from burning or exploding if over-charged/over-discharged/or exceeding discharge rate, and those are the LiFePO4 and Lithium Nano's.

The problem with using them with inverters is voltage. As you probably know battery voltage that's usually thrown around is merely for classification. The actual operating voltage is going to be a range. Most inverters have over-voltage protection set at 15v. Some say 16v but it's still usually 15v. Most lithium cells are 3.7v each, so the two closest pack voltages will add up to 11.1 or 14.8v. Now that's not what your pack will actually measure. A 3.7v cell will measure 4.2v when fully charged so your 14.8v pack will read 16.8v when it comes off the charger, so you'll have to apply a load to it to bring it down to an acceptable level before the inverter will accept it. And an 11.1v pack will start as 12.6v but it will quickly drop to the inverters low voltage thresh hold well before it is actually depleted.

That leaves us with the LiFePO4 cells. They're 3.2v nominal, fire and explosion safe. They have high energy density and high discharge rates, as well as low self discharge rates. They also have 4 times the cycle life of lithium ion and up to 10 times the cycle life of lithium polymer. They can be bought for a little over $1/wh online.

I still prefer NiMH as my workhorse, but I'm slowly coming around to the idea of LiFePO4 as prices drop.

[Sunking]

There are two main ways that NiMH "smart chargers" determine a NiMH battery is fully charged.

You are preaching to the choir.