Thread: Battery life?
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Unread 05-05-2011, 00:58
Dale(294engr]'s Avatar
Dale(294engr] Dale(294engr] is offline
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Re: Battery life?

Here is a 50A load battery characterization on our 2006-1 SLA conducted Jan 19, 2006 (note: typo on graph)

DMM is directly connected to battery terminals (so excludes all connector & wiring resistance voltage drops)

Vterm recorded & plotted every 10 seconds until Vbatt drops to 7.0v

started at -3 minutes with .5A fan preload to illustrate 'soft' voltage area between ~13.3 - 12.6v becoming 'hard at ~12.6v at high discharge rates

'hard' useful robot current occurs at .018 ohm avg internal battery resistance
then plummets quickly

note: Battery Terminal voltage drops 1v for each 50A of load

4 CIM drives with friction belt contact to floor (or pushing opponent)
start up currents can hit 400-500A for few hundred milliseconds resulting in
an 8 to 10 v drop from the 'hard' Vterm!! (excludes connector wiring Vdrops

(dynamic current: use Oscope across .001ohm shunt to verify!! scale 1A/mV
or use 120A CB as shunt in pinch for this, its R closely controlled ~.00096ohm
also useful for rough robot DC current: DMM 200mV scale across 120A CB)

400A: 12.6v-8v = 4.6v to keep CRIO, Radio etc alive else it resets!!
500A: 12.6v-10v = 2.6v CRIO, Radio etc reset!!

Advice:
Charging: do it ASAP after every use, float charge ALL other times
abide by mfr spec max fast charge, (guaranteed to do no harm)

Matches: new battery every time
....... used batteries =Increased Rint reduces max bot fps + pushing power
Fast charge (~5A) gets you to only ~85% upon completion
..remaining 15% float mode takes >=3 hours!! use your Auto dual rate charger and leave it on. have one or two batteries on charge overnight for critical matches for speed power edge needed.

Load curve reveals much about State of Health of a Battery
..expected life, speed and motor torque loss as time passes due to Vdrop resulting from higher internal battery resistance as it gives up its energy

Note upswing of voltage after heavy load depression is Normal
.. internal I^R generates heat: chemical reaction result: increasing voltage for a short period! isn't science wonderful?

if robot consumption expected closer to 100A average run Batt test at 100A!

main challenge: find Load Resistor(s) to take heat for about 10minute for 50A .25ohm test, or 5 min for .125ohm 100A test

(swapmeets often sell 200W AL heat sink 1% WW resistors inexpensively
series parallel as required P=I^2R size for 12.6v initial voltage-1v / 50A)

Dale.. 2 attachments data, graph + imbedded calcs as Rint changes

note: my email has changed from that on the attachments
now DaleScience@aol.com
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