DIY Manual

34 | P a g e 10-543-1 REV A
DIY Manual
Let’s do some math! Assume you bought a single 12V, 100Ah battery. That gives you 1200Wh
(12V x 100Ah) of storage. Let’s next assume that you need to grow your bank 6 months later,
and you want 4800Wh of capacity, so you go and buy 3 more batteries. You now have four 12V
batteries for a 12V system and your batteries must be wired in parallel making a 4-string bank
(See Figure 4). And remember, more than 3 strings are not good (for flooded or sealed batt
types).
Let’s go back in time … You initially buy two 6V batteries, each rated at 370Ah, instead of the
single 12V battery at 100Ah. Each 6V battery has 2220Wh (6V x 370Ah) of capacity for a 4400Wh
(2 batteries x 2200Wh) bank, consisting of two 6V batteries in one series string (See Figure 5).
Thus, an original purchase of two 6V, 370Ah batteries is almost the same in watt-hours as four
12V, 100Ah batteries (4400Wh versus 4800Wh). The upfront costs, though, are why most DIYers
buy incrementally, but doing so is usually not advantageous in the long run. A typical 6V, 370Ah
battery costs about $245 - $300, or $490 - $600 for two. Four 12V, 100Ah batteries usually cost
about $100 each, or about $400 compared to $490 - $600.
Bottom line … Plan ahead and buy today what you need for tomorrow to avoid getting in to a
high string count battery bank.
Amp-hour Capacity
A battery bank’s total amp-hour (Ah) capacity equals the amp-hour sum of the strings. In Figure
4, each battery has a 20-hr rating of 100Ah. In a parallel circuit, voltage stays the same and
current adds. So, for the bank in Figure 4, we have four 12V strings at 100Ah each string.
Therefore, Figure 4 shows a 12V bank with an amp-hour capacity of 400Ah (4 strings x 100Ah).
In a series circuit, current stays the same, and voltage adds. Looking at Figure 5, we have two 6V
batteries in series which creates a 12V bank. Each battery is rated at 370Ah; since the two
batteries are in series, the current stays the same, therefore Figure 5 shows a 370Ah bank at 12V.
Let’s check the math ….
Figure 4: 4 strings at 12V at 100Ah. 4 x 12 x 100 = 4800Wh.
Figure 5: 1 string at 12V at 370Ah. 1 x 12 x 370 = 4440Wh.
SERIES – Positive to Negative
PARALLEL – Positive to Positive; Negative to Negative