Alternator Charging Capacity
Test Procedure
Version 4
CAT Electronics
Starting and Charging Systems
Last Updated: 10 May 2012
Author: Sam Revach
1
Caterpillar
Confidential: Green
Overview - Alternator Charging Capacity
Purpose: Test alternator charging capacity.
This test will verify proper sizing of alternator and battery charging capacity.
The procedure has three elements with testing at,
1.
2.
3.
Install a current probe on the battery output cable and alternator output cable.
Install volt meter across battery packs and alternator terminals. (see diagram)
2.
Measure machine total load by incrementally turning on and measuring all machine loads, starting
with ‘Key Switch’ on, (see attached sample).
Machine total load
3.
Partially discharge batteries to ~(80%) state of charge {~(24.9V)} by turning on all possible electrical
loads for (20 to 30) minutes without engine running. Monitor voltage and current draw from
batteries.
a. This ensures that during testing the batteries will accept a charge and alternator will be at full
output.
b. Orient battery current probe so that the polarity of the reading is ‘positive’. In this case,
batteries are discharging when reading is positive, and charging when reading is negative.
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Procedure – Alternator Charging Capacity
4.
Start engine and operate at low idle. Ensure all possible electrical loads are on.
Record instantaneous & simultaneous current and voltage measurements.
Notes:
a. Since values change rapidly as batteries recharge, measurements should be taken quickly and
simultaneously from alternator & battery. (Two people are needed to record the data. One of them
will give the signal to record).
b. Machine loads = (Alternator current) + (Battery current)
Keep in mind that the batteries may be discharging, ‘adding to’ (+), OR charging, ‘taking from’
the alternator (-).
5.
Run engine up to high idle and repeat measurements (as above).
6.
While at low idle, take (3) additional simultaneous measurements of alternator & battery current at (2)
minutes intervals in ‘steady state’ conditions (i.e. after the engine idled for a few minutes).
7.
If engine has a hibernate mode at very low engine speed, then allow engine to drop down into
hibernate mode and take measurements from alternator and batteries as earlier.
Alternator can meet machine electrical load* + customer loads** at all engine speeds.
Alternator can charge batteries at 10A. per parallel set of batteries.
* Allowance should be provided for fully loaded transmission ECM.
** Customer loads, such as, fire suppression system, added radios - required for operation in quarries etc. Such loads may
vary from 5A to 20A, as needed.
Note: If battery current measure is positive polarity, then the alternator is not meeting demand of machine electrical
loads and batteries are being discharged.
Machine Loads
Power
Distribution
Battery current
Probe
Battery voltage
Alternator
Current probe
Frame
(+)
(+)
(-)
Alternator voltage
Alternator
Battery
Test complete.
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