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Re: (ET) Fully charged pack voltage
On Thu, 23 Apr 2009, David Roden wrote:
It's important to make the distinction between open-circuit voltage
voltage, with nothing connected, 12 to 14 hours after the charge has been
stopped) and on-charge voltage (the voltage with the charger connected).
When you read that a battery should be charged to around 2.4 to 2.5 volts
per cell, that is the on-charge voltage.
However, that by itself is NOT enough information! A battery isn't charged
until it reaches the specified voltage AND the charge current has fallen to
about 2% of the battery's 20-hour amp-hour capacity, expressed in amps.
For example, you should charge a 240 amp-hour golf car battery until its
charge voltage reaches around 7.35 volts (some say as much as 7.5 volts)
the current falls to 4.8 amps.
That is very useful info. In principal, this should allow a person
to use a Kill-a-Watt meter where the charger plugs into 120V to make a
decent estimate of when the batteries are charged (and get by with just
the basic charger that comes with the tractors). 4.8 Amps getting dumped
into a pack at 44.1 Volts (7.35 V per battery) is 211.7 Watts. Of course,
the charger isn't 100% efficient - I would guesstimate maybe 90%
efficient? So, if the power being drawn from the wall outlet drops to 235
Watts, and the charging voltage has risen to ~44 V, then we could assume
the pack is fully charged (for 240 Amp-hr batteries).
Not as convenient as a well controlled charger of course, but it
could give a way to know when the pack is "done" for those just using the
regular timer. Of course, my guess of the charger efficient is just that,
a guess - it would be better to measure the current getting dumped
directly into the batteries.
I suggest reading this document :
Open-circuit voltage is an inaccurate indicator of state of charge, but
all you have for sealed batteries. If you're using flooded batteries,
there's no reason to use voltage, because a hydrometer is much more
A voltmeter is just much more convenient. With specific gravity -
shouldn't it vary from battery to battery? (battery maker to battery
maker, actually) My understanding is that some battery makers put in
"extra" sulfuric acid, which would shift the SG readings.
PS - I say again : in my experience, more batteries die from overcharging
than from undercharging.
That makes sense.