Thomas Shay wrote:
>>> A 120 volt pack isn't empty when its voltage drops to 105. There's
>>> nothing sacred about 105 volts. A conservative driver wanting to
>>> be kind to his batteries would do well to avoid frequent drops
>>> below 105. A less conservative but still reasonable limit is 90
>>> volts for short bursts of acceleration or hill climbing.
Peter VanDerWal wrote:
>> We were talking about a constant current draw. By definition, a
>> lead-acid cell is empty (at a given current) when its voltage drops
>> to 1.75V. A 120V (nominal) pack has 60 cells, so when used at a
>> constant current, it is empty when it reaches 105V.
Both are true. The important point is that batteries don't suddenly go
dead, like an empty gas tank. The rules for lead-acid batteries are:
1. The lower the battery's state of charge, the lower the voltage.
- 2.16v/cell = 100%, 1.99v/cell = 0%
2. The more current you draw, the lower the voltage.
- a flooded 6v golf cart battery's internal resistance is about
0.005 ohms at full charge and 77 deg.F
- a sealed AGM 12v Optima battery's internal resistance is about
0.002 ohms at full charge and 77 deg.F
- voltage under load is about Vload = Vnoload - (Amps x Resistance)
3. Internal resistance goes up as the battery runs down
- it can be 2 times higher at 50% state of charge, and
nearing full discharge, the battery is nearly an open circuit
4. As the battery gets colder, internal resistance goes up and amphour
capacity goes down, approaching an open circuit and zero amphours
at -40 deg.
5. To murder a lead-acid cell, pull it below 1.5v.
- but batteries use many cells in series, and they aren't matched
- so 1.5v x 6 = 9v is not a safe minimum for a 6-cell 12v battery
because one cell might be at 1v when the rest are at 1.6v.
So, the lower you go in voltage, the luckier you had better be. 9v might
be OK if your 12v battery is new, in good shape, and has been recently
equalized so all cells are as equal as they're going to get. But it will
kill the weakest cell of a battery that is older, not in perfect shape,
and hasn't been equalized.
Steve Clunn wrote:
> "There will be no need for war as we have greatly understamated the
> power of the Hood ornamant"
> Steve Clunn (lee are you still with me :-(
Interesting idea, but wrong topic, Steve. Let's help the EV Newbie with
his questions here.
--
Lee A. Hart Ring the bells that still can ring
814 8th Ave. N. Forget your perfect offering
Sartell, MN 56377 USA There is a crack in everything
leeahart_at_earthlink.net That's how the light gets in - Leonard Cohen
