Electronic – Advantages of chaining another battery to 130Ah AGM deep cycle 12V battery

battery-charginglead-acidsolar energy

I have a 130Ah AGM deep cycle 12V battery for camping connected to 2 x 100W solar panels. I have been testing the system out to see whether it will sustain what I require. I think I’ll require around 100Ah of charge per day which would approximately discharge the battery to 25% SOC.

How much charge can my solar panels theoretically replenish if I can access say 6 quality sun hours in a day?

Does the wattage of the solar panels represent the amount of charge that can be supplied to the battery per hour? If so, I imagine I could generate 1200 Watt Hours or 100 Amp Hours in a day which would be enough to replenish the amount of charge that I wish to consume. Is my math correct?

If I didn’t use more than 100Ah of charge per day, would there still be an advantage in terms of battery longevity by chaining another identical battery to my existing one to double the capacity?

If I wanted to use more than 100Ah of charge then I would require a second battery so as not to discharge the first battery to less than 25% SOC. However, would my solar panels be capable of regenerating enough charge to make adding the second battery viable? Or would there be other advantages to add the second battery in terms of operating on average at a higher SOC even if I were unable to regenerate all of the charge that I had consumed in a day?

Best Answer

You need more than 150% of the power you use if you are using very low resistance parts. Otherwise you will need more. In the real world, typical solar panels don't actually produce what they are advertised to produce. In the real world batteries need more power than used to replenish their power with. Only the purest, least resistance batteries give extremely efficient recharge rates. Most produce heat and some swelling in the battery (AGM) which is loss of power. And you need a higher level of power offered to the battery than it currently has in order to charge it. I recommend double your perceived power achieved. Remember, only 1/3 the day is usable daylight typically, so if you want to use the battery after dark, the numbers get alot bigger. Never discharge a 12v battery under 12v whenever possible, and ABSOLUTELY NEVER allow a battery to remain in a discharged state for any period of time if possible. I have assumed lead acid batteries here, but they are all similar in the problems associated with them.

90.5% @0–100% SoC

for electrolyte type A

89.1% @0–100% SoC

for electrolyte type B

I read reverse calculated this from graphs in a non-public measurement report where 2 different types of battery electrolyte (solid/gel) technologies were compared for a 0 to 100% SoC (state-of-charge).

Battery measurements were taken by a technical university, at 21ºC room temperature during 1 month, the year 2013, comparing the results from 150-180 discharge/charge cycles. Batteries under test were all in the range between 5 and 10Ah.

charging curve: CC-CV
charge to discharge point: V = 14.375V and I < 0.30A
discharge to charge point: 0% SoC
resting period: not specified
0% SoC was defined as discharge voltage reached 10.8V (1.8V per cell)
100 SoC was defined as charge voltage reached 14.375V (2.396V per cell)
charge was done at a rate of 0.3C (+/- 0.05C)
discharge was done at a rate of approximately 0.1C (fixed resistor was used)

The result between charging and discharge power was (remarkably) about 1.6Wh for both battery types, where one battery had a remaining capacity of approximately 40Wh and the other 20Wh.

Some data read back from the graph (resulting in lower accuracy) for electrolyte technology A:

Cycle   Charge [Wh]   Discharge [Wh]
1       100           91
2       100           88
3       100           86
4        97           81
5        92           76
6        86           73
7        81           69
8        78           67
9        74           64
10       73           63
11       70           61
12       69           59
13       65           56
14       63           55
15       61           52

136      44           40
137      44           40
138      44           40
139      45           40
140      45           40
141      43           40
142      44           40
143      45           40
144      44           40
145      44           40

According to my opinion the last measurements are more significant than the first measurements due to the initial formation process. Average last 10 cycles power charged into the battery is 44.2Wh. Average last 10 cycles power discharged from the battery is 40.0Wh.

Some data read back from the graph (human graph point via line to value translation) for electrolyte technology B:

Cycle   Charge [Wh]   Discharge [Wh]
1        85           75
2        83           72
3        80           70
4        78           69
5        76           67
6        76           67
7        75           67
8        75           67
9        74           66
10       74           66
11       73           65
12       73           66
13       72           66
14       72           65
15       73           64

136      25           22
137      25           22
138      25           22
139      24           22
140      25           22
141      24           22
142      25           22
143      25           22
144      25           22
145      24           22

Average power charged into the battery during last 10 cycles is 24.7Wh. Average power discharged from the battery is 22.0Wh.

Electronic – Switching between two sources of power to a battery

You can use a Schottky diode in series at the charger and solar panel and then connect them both to the battery. The drawback is a ~0.3V drop on the diode. Not a problem in case of the charger, but will reduce the power output of the solar cell. On the other hand - smaller PCB area, cheaper and more reliable.

Best Answer

Related Solutions

Electronic – the charge-discharge watt hour (Wh) efficiency for lead acid batteries at different SoC and charge current 10-35A/100Ah C10

90.5% @0–100% SoC

89.1% @0–100% SoC

Electronic – Switching between two sources of power to a battery

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