A hydrometer or digital voltmeter can be used to test the battery to see if it is fully charged. A hydrometer checks the specific gravity of the battery fluid (electrolyte). It will be heavier than water. There are cheap hydrometers for sale ($5+-) that use little balls or something similar and are equipped with a chart to tell you what the cell condition is, and the charge condition.
WARNING! BATTERY ELECTROLYTE CONTAINS A HIGH CORROSIVE CALLED SULFURIC ACID.
Be sure to wear goggles and rubber gloves when handling battery fluids. Battery acid will eat through clothes (and paint). Polyester is resistant to battery acid. If you have a spill, wash everything with a mixture of baking soda and water, as the baking soda will neutralize the acid. Be sure not to get the baking soda mix into the battery. It will destroy the battery. Some batteries are maintenance free and cannot be checked with a hydrometer. These batteries can be checked with a digital voltmeter.
WARNING! VERY FLAMMABLE! HYDROGEN GAS IS GIVEN OFF WHEN BATTERY IS CHARGING. DO NOT HAVE ANY FLAME OR SPARK NEAR BATTERY.
WARNING! VERY EXPLOSIVE! UNVENTED HYDROGEN GAS IS GIVEN OFF WHEN BATTERY IS CHARGING. Loosen the battery caps if charging the battery.
Batteries that have caps over the cells can be checked with a hydrometer. Check the fluid level in the cells to be sure it is not low. Take the cap off and look into the cell. You should be able to see the fluid level just below the lowest lip of the opening.
If the fluid level is low enough to see the battery plates, then distilled water must be added. This could cause the battery to be low in charge or dead.
If the fluid level is down, a hydrometer reading cannot be made. USE DISTILLED WATER ONLY! Tap water will have too many other minerals and will affect operation and life of the battery. DO NOT OVERFILL the cells. To the right you will see a couple of fillers used to put distilled water into a battery.
|
Percentage of Charge |
Specific Gravity Corrected to |
Open-Circuit Voltage |
||||
|
6V |
12V |
24V |
36V |
48V |
||
|
100 |
1.277 |
6.37 |
12.73 |
25.46 |
38.20 |
50.93 |
|
90 |
1.258 |
6.31 |
12.62 |
25.24 |
37.85 |
50.47 |
|
80 |
1.238 |
6.25 |
12.50 |
25.00 |
37.49 |
49.99 |
|
70 |
1.217 |
6.19 |
12.37 |
24.74 |
37.12 |
49.49 |
|
60 |
1.195 |
6.12 |
12.24 |
24.48 |
36.72 |
48.96 |
|
50 |
1.172 |
6.05 |
12.10 |
24.20 |
36.31 |
48.41 |
|
40 |
1.148 |
5.98 |
11.96 |
23.92 |
35.87 |
47.83 |
|
30 |
1.124 |
5.91 |
11.81 |
23.63 |
35.44 |
47.26 |
|
20 |
1.098 |
5.83 |
11.66 |
23.32 |
34.97 |
46.63 |
|
10 |
1.073 |
5.75 |
11.51 |
23.02 |
34.52 |
46.03 |
The chart at the left gives the specific gravity and volt reading of the charged cells. 
An example of a simple, cheap hydrometer is pictured at the right. Fill the hydrometer several times before taking a final reading. If any of the cells have a .05 variance in the hydrometer reading, the cell with a lower reading is weak. Each cell should have nearly the same reading, whether the battery is charged or discharged. Also, the chart readings are for electrolite at 80 degrees F.
If you are using a non-temperature adjusting hydrometer, use the chart below to add or subtract to the reading.The picture at the right is a typical volt meter. The same chart at the left has the voltage readings to tell the state of the battery charge. Connect the red probe of the volt meter to the positive battery post and the black voltmeter probe to the negative post. If the volt meter shows a negative reading, the volt meter probes are connected to the wrong battery post. Like the hydrometer reading, if the temperature is lower or higher, the volt reading has to be adjusted. Use the chart below for making adjustments to the readings according to temperature. If the battery is suspected to be fully charged but has a volt reading of 10.5, there is the likelyhood that there is a shorted cell in the battery.
|
Electrolyte Temperature Fahrenheit |
Electrolyte Temperature Celsius |
Add or Subtract to Hydrometer's SG Reading |
Add or Subtract to Digital Voltmeter's Reading |
|
160° |
71.1° |
+.032 |
+.192 |
|
150° |
65.6° |
+.028 |
+.168 |
|
140° |
60.0° |
+.024 |
+.144 |
|
130° |
54.4° |
+.020 |
+.120 |
|
120° |
48.9° |
+.016 |
+.096 |
|
110° |
43.3° |
+.012 |
+.072 |
|
100° |
37.8° |
+.008 |
+.048 |
|
90° |
32.2° |
+.004 |
+.024 |
|
80° |
26.7° |
0 |
0 |
|
70° |
21.1° |
-.004 |
-.024 |
|
60° |
15.6° |
-.008 |
-.048 |
|
50° |
10° |
-.012 |
-.072 |
|
40° |
4.4° |
-.016 |
-.096 |
|
30° |
-1.1° |
-.020 |
-.120 |
|
20° |
-6.7° |
-.024 |
-.144 |
|
10° |
-12.2° |
-.028 |
-.168 |
|
0° |
-17.8° |
-.032 |
-.192 |
Some data collected from these sites.
uuhome
global
trojan
icarumba
interstate
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