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I was wanting to buy a 12v battery that could supply 8A for around an hour. I thought I knew what I was doing and so ordered the following 12v 10Ah Deep Cycle battery.

Yuasa REC10-12 BATTERY Deep Cycle Cyclic SLA 12V 10Ah

I chose deep cycle as from my understanding, they cope better with running the battery down low. And I chose 10Ah as I thought that meant it could supply 10 amps for up to an hour.

However, upon testing this, it lasted about 10 minutes :(

Having done a little more research, I have found out the the 10Ah stated is not as simple as I thought it was, and you have to consider the Nominal Capacity specs to figure out how long you'd actually get.

When I look at other battery data sheets, they specify this data clearly, for example here :

http://www.power-sonic.com/images/powersonic/sla_batteries/ps_psg_series/12volt/PS-1220_11_Feb_21.pdf

Nominal Capacity

 20-hr.    (125mA to 10.50 volts)      2.50 AH
 10-hr.    (220mA to 10.50 volts)      2.20 AH
 5-hr.     (400mA to 10.20 volts)      2.00 AH
 1-hr.     (1.5A to 9.00 volts)        1.50 AH
 15-min.   (4.5A to 9.00 volts)        1.13 AH

But having looked at the datasheet for the battery I bought, it doesn't show the Nominal Capacity? could anyone help me to understand from the datasheet, how I would work out the expected runtime at 8A?

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  • \$\begingroup\$ How (and how long) did you charge the battery before testing the run time? \$\endgroup\$ – Jonathan S. Nov 11 '17 at 19:48
  • \$\begingroup\$ I charged it with a car battery charger until the 'charged' light came on. I did it over night, so not sure how long it took. \$\endgroup\$ – wforl Nov 11 '17 at 19:54
  • \$\begingroup\$ What's the output current of that car battery charger? Is it one that's designed to charge large (~100Ah) car batteries? If yes, it might have killed the battery by overcharging it. According to the datasheet that Transistor helpfully provided, it should've lasted at least 45 minutes if the battery wasn't damaged. \$\endgroup\$ – Jonathan S. Nov 11 '17 at 19:59
  • \$\begingroup\$ hmm - I don't have it to hand, but can check when I get home. It's this one though halfords.com/workshop-tools/garage-equipment/… \$\endgroup\$ – wforl Nov 11 '17 at 20:03
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    \$\begingroup\$ Ouch! The description of the charger says "not suitable for AGM batteries", which is exactly the type of battery you have. \$\endgroup\$ – Jonathan S. Nov 11 '17 at 20:06
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With the information you added in the comments of your question, I'm pretty sure now that the charger you used killed the battery by overcharging it.

It says "Please note - Not suitable for AGM batteries" in its description and you've got a relatively low capacity AGM battery. The charger probably kept the battery at a voltage of more than 14V for the whole night, overcharging it dramatically.

Edit: The description of the charger also says that it's "not suitable for long-term connection", indicating that it does indeed just keep pumping current into the battery even when it's full.

You should use a 16Ah deep-cycle 12V battery with a suitable charger. You also shouldn't let the battery charge over night but stop the charging immediately when it's full. (Or get a charger that does this for you)

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  • \$\begingroup\$ Well crap. I saw 'suitable for lead acid battery types' on the front of the charger, and thought it was ok. What a fool :( \$\endgroup\$ – wforl Nov 11 '17 at 20:21
  • \$\begingroup\$ @wforl It happens. Just be sure to buy a slightly bigger battery than what you had (16Ah should do) and a good charger. Look for an "AGM" or "gel type" battery charger and don't buy a super cheap one. The charger should have at least some intelligence to detect that the battery is full and cut off the charging current completely (or go into floating charge mode). \$\endgroup\$ – Jonathan S. Nov 11 '17 at 20:26
  • \$\begingroup\$ Would something like this be suitable? uk.rs-online.com/web/p/battery-chargers-lead-acid-automotive/… \$\endgroup\$ – wforl Nov 11 '17 at 20:35
  • \$\begingroup\$ @wforl Yes, this one should work. It has overcharging protection and trickle charge functionality and is made for small-ish AGM batteries. It'll take about a day to fully charge your battery with that charger which is about right and will cause less wear of the battery than charging it quickly within a few hours. \$\endgroup\$ – Jonathan S. Nov 11 '17 at 20:41
  • \$\begingroup\$ That charger has a stated range of 2-24Ah, if I choose to go to a higher Ah battery in the future (26Ah or 36Ah for example) will this charger still work, only at a slower rate? or will it not function propertly? \$\endgroup\$ – wforl Nov 11 '17 at 21:04
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You didn't link to the datasheet but I found one on Farnell's site. It shows the data you seek on the front page.

enter image description here

Figure 1. Nominal capacity at various discharge rates.

Even better, they have a graph.

enter image description here

Figure 2. Discharge curves for the battery.

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  • \$\begingroup\$ Oh what!!!?? .. I was looking on the Yuasa site, which only gives this yuasa.co.uk/batteries/industrial/rec-vrla-cyclic-use/… ... thanks! \$\endgroup\$ – wforl Nov 11 '17 at 19:57
  • \$\begingroup\$ So assuming I was drawing 10A, so 1CA? .. it dips below 12V at the 10 minute mark. So at 8A, it's not going to last much more than 10 minutes? \$\endgroup\$ – wforl Nov 11 '17 at 20:07
  • \$\begingroup\$ @wforl The battery isn't drained at 12V yet, you can discharge it until it has about 10V. \$\endgroup\$ – Jonathan S. Nov 11 '17 at 20:13
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the batteries appear to be labeled per 20Hr discharge cycle. (gotta love those advertising people)

see this doc titled Sealed Lead-Acid Batteries Technical Manual, page 9

http://www.power-sonic.com/images/powersonic/technical/1277751263_20100627-TechManual-Lo.pdf

i think that it shows that you need an 18AH battery for the runtime you need.

here is a list of the products http://www.power-sonic.com/pdc_deep_cycle_series.php

from there, I guessed at part number PDC-12260 http://www.power-sonic.com/images/powersonic/sla_batteries/20140418-PDC-122601.pdf

page 2 ... top left chart ... the output voltage should be around 11.8V after 60 minutes of 8A discharge current (at 25°C ambient temperature)

enter image description here

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  • \$\begingroup\$ It'd be better if you embedded more of the info from the links into the answer in case Power-Sonic reorganizes their website and breaks your links :p \$\endgroup\$ – ThreePhaseEel Nov 11 '17 at 20:41
  • \$\begingroup\$ @ThreePhaseEel, you are absolutely correct about the links changing. there is too much data to post, so i included the name of the document and the part number of the battery, so that anyone can do a web search to find it if links change. \$\endgroup\$ – jsotola Nov 12 '17 at 1:21

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