Weigh of different 12V 7Ah batteries - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: April 26, 2016, 05:18:26 am
content: I thought I was imagining things for a moment, so I weighed two different 12V 7Ah batteries here at work, and was surprised at the difference. One was exactly 1/3 heavier than the other. We buy them a palette load at a time so I don't know if there was a price difference. Would have been interesting to measure the actual capacity of the older (heavier) one when it was new. The lighter ones we have just received. The actual capacity is not an issue for us, provided it is not like half the stated amount, but it does make you wonder how many corners are being cut.If you have some, weigh them and post the results.Edit - Yuasa 12V 7Ah is 2.65kg according to the catalog.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 26, 2016, 06:34:29 am
content: They will have same C20 capacity but at faster discharge will be differenceLook at datasheetsHeavier use thicker electrodes so have smaller internal resistance and should last few more cyclesYausa NP7-12 have 25 mOhms with 210A short circuitwhile Neuton NP1270 have 40 mOhms with just 97A short circuitand lighter will have even worse

author: johansen
author_rank: Super Contributor
timestamp: April 26, 2016, 07:13:20 am
content: Short circuit current is usually 30 to 40 times the amp hour rating, across all battery types, agm or vehicle starting batteries.also, i don't remember the manufacturer but the last datasheets i went through, the impedance listed in the datasheet matched the short circuit amps by ohms law. 12.56 volts/impedance = s.c. ampsthe numbers Miyuki posted are off by a factor of three for that to be the case.---no way will those two batteries have the same c/20 amp hours.I have some (allegedly) 9ah cells and they are 2.8kg.if you have a legit 1.8kg, 7 ah battery you might have a new world record for watt hours per kilogram.

author: woodchips
author_rank: Frequent Contributor
timestamp: April 26, 2016, 08:01:26 am
content: Isn't most of the lead actually in the frames holding the paste and the other interconnections? So if they were all halved in size then there would be a significant weight loss. If you never need very high currents then could be a way to save weight.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 26, 2016, 09:14:26 am
content: I found NP1270M model datasheetBattery Model NP1270M 12V7AHDesigned Floating Life 3?5 YearsCapacity (25?)20HR(0.26A,10.5V) 5.2AH10HR(0.492A,10.5V) 4.92AH5HR(0.884A,10.5V) 4.42AH1HR(3.01A,10.5V) 3.01AHApprox. Weight 1.88KgWhat a great batteryThere is also model NP1270 with weight 2.25Kg and it is real 7AH C20This look like official fake

author: Ian.M
author_rank: Super Contributor
timestamp: April 26, 2016, 10:17:53 am
content: Theelectrochemistry of a Lead acid cellat each plate is such that two electrons are transferred per atom of Lead, but as both plates are needed, to store two electrons it needs PbO2+ Pb +2H2SO4. Adding up the atomic masses:Pb: 207.2 *2 = 414.4O: 16 *10 = 160H: 1 *4 = 4S: 32 *2 = 64Total: 642.4 or 321.2 per single electron.Additionally, the electrolyte of a fully charged battery is 5.2 molar, so for every 5.2 moles of acid there is 1 litre of water. That's approximately 10.7 molecules of H2O per molecule of acid so add 192.3 for the molecular mass of the required electrolyte, giving a result of 513.5 per electron. Divide by Faraday's constant and multiply by 3600, and you get 19.5 g/AH. Multiply by 6 for a 12V battery, and by 7 for 7AH, and you get just under 805g for the chemically active parts of the battery. (unless I've FUBARed the maths)However, you need a lead matrix to support the active electrodes, more electrolyte to saturate the separator and surround the electrodes + the case etc. so 19.5 g/AH/cell is only the theoretical limit.Additionally to the problems with cell internal resistance mentioned above, the lead matrix of the plates tends to corrode during the life of the battery, loosening the electrode material in its pockets so it sheds, reducing the capacity and also increasing the internal resistance, so the expected life of a battery (optimised for a particular application) is directly related to the thickness of lead in its plates and thus its mass.

author: SeanB
author_rank: Super Contributor
timestamp: April 26, 2016, 04:55:45 pm
content: 1.8kg one is not going to last long, as they have shaved off too much lead mass, probably by simply using one plate less in each side of the cells. I just tried an Enertec and a generic one, which were 2.2 and 2.0kg each.I have seen the real fakes, where the construction inside was to have a single plate for each electrode, and then a separator, the rest of the internal volume being taken up by a sheet of expanded polystyrene filling the cavity. Marked with the nominal voltage and the standard capacity for that sdize unit, but in reality they were around 10% of the capacity, plus they were suspiciously lighter than the real ones. Operational life was around 3 months in any case, which was a lot poorer than the genuine ones.