2.5.2. Dimensions and weight
Battery type
V
Ah
kWh
Volume dm 3
Weight kg
Specific volume Wh / dm 3
Specific weight Wh / kg
Start
12
100
1.2
16
28
75
43
Spiral-cell
12
60
0.72
8.5
17.2
81
42
Semi-traction
12
200
2.4
33
60
73
40
VRLA AGM battery Traction (tubular-plate)
12
230
2.8
33
62
85
45
24
1000
24
280
770
85
32
VRLA-gel Sonnenschein Dryfit A200 VRLA-gel Sonnenschein Dryfit A600
12
200
2.4
33
70
72
34
24
1500
36
600
1440
60
25
This table very clearly shows how heavy and cumbersome batteries are.
Coming back to the comparison in section 2.1: Compared to the energy released by combustion of diesel fuel, for example, batteries are simply no rivals. Burning 10 litres (weight 8.4 kg) of fuel generates approx. 100 kWh of thermal energy. So when consuming 10 litres of diesel fuel a diesel generator with an average efficiency of 20% will be able to generate 20 kWh of electric energy. This is the energy needed to charge a 24 V 700 Ah battery. Such a battery has a volume of 300 dm 3 (= 300 litres) and weighs 670 kg!
Another telling comparison is heating water. Bringing 1 litre (= 1 kg) of water to the boil in an electric kettle requires 0.1 kWh. To supply the required 0.1 kWh, approx. 4 kg of battery is needed!
2.5.3. Effect on capacity of rapid discharging
The capacity of a battery is dependent on the rate of discharge. The faster the rate of discharge, the less Ah capacity will be available. This is related to the diffusion process (sect. 2.2.3). In general the rated capacity is quoted for a discharge time of 20 hours (discharge current I = C / 20).
For a 200 Ah battery this means that the rated capacity can be delivered at a discharge current of 200 Ah / 20 hours = 10 Ampères.
With a discharge current of 200 A the same battery becomes “flat” far sooner. For instance a 200 Ah gel battery then has an effective capacity of only 100 Ah and therefore becomes flat after 30 minutes. (see also chapter 3: The battery monitor).
The following tables give an impression of the capacity as a function of the discharge current.
The 2 nd column of the first table gives the rated capacity as quoted by the manufacturer with the associated discharge time. Often this is 20 hours, but it can also be 10 hours or 5 hours.
The tables show how capacity falls off steeply with increasing discharge current, and that AGM batteries (especially the spiral-cell battery) perform better than gel batteries under high discharge currents.
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