Newbie battery questions

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NeilK
Posts: 10
Joined: Sat Oct 01, 2011 10:28 am
Location: Orkney

Newbie battery questions

Postby NeilK » Wed Oct 05, 2011 7:43 pm

Hi,

I need to replace the 6 lead acid batteries in the Matiz conversion I bought and am feeling my way in here. A few questions are rattling round in my head and I'd be glad of some steers if possible.

1. Sealed v Low Maint Lead Acids. Is there much to choose in terms of performance/reliability etc? Access is not an issue, but not sure if there are any specifics that relate to using Gel batteries for the comparatively high currents for traction uses. Or is a liquid electrolyte better? (and while we are at it, what does AGM stand for?)

2. The car has 1 Trojan 130Ah & 5 others that I cannot find any name on. I suspect they are US Battery units from the handle and terminal configuration. Somewhere in the back of my head I have it that changing one battery from a set is a bad idea. I was thinking that the rule is change one, change them all........... but I may have made that up. Can anybody comment on this please?

3. I am looking at the US31TMX Sealed Deep Cycle Battery which shows as a 130Ah unit on some sites, but I presently find the tecnical spec is confusing. It says Amp Hours (20Hr rate 130mins) then says 25A for 227 min and 75A for 58 mins. What does all this mean? It looks to me as though this is saying: 75A for about 1hr ~ 75Ah.... or 25A for 227 mins (nearly 4 hrs) ~100Ah...... if you can point me at something definitive I will get my head around it.

4. Similarly there are references on the site to rates of 0.5C and 1C..........

I have read a lot of the battery posts on the site and know more than I did, but would be glad of a pointer to a glossary on some of this if it is there. :?

Thanks

GregsGarage
Posts: 870
Joined: Tue Apr 01, 2008 5:27 pm
Location: Galashiels, Scottish Borders
Contact:

Re: Newbie battery questions

Postby GregsGarage » Mon Oct 10, 2011 1:46 pm

Hi NellK,

Since nobody else has jumped in with an answer I will start the ball rolling.

1. Easiest will be to fit the same as you already have. Liquid electrolyte batteries are the most forgiving because you can top up the water if they are overcharged, and this is usually how you balance them. Gel and AGM (absorbed glass matt) are true sealed batteries with no liquid electrolyte, so if you overcharge them they vent gasses and dry out, ruining the battery.

2. its o.k. to change one battery, you just need to make sure you don't let any batteries overdischarge. If I remember correctly its around 10 volts for a lead acid. The problem with having different age batteries, is the new ones will hopefully be better than the old ones and so trying to use just pack voltage as an indication of SOC can be misleading. Something like the Paktrakr will allow you to monitor individual batteries. If all the batteries are the same age and well matched then pack voltage may be all you need.

3. Look up Peukert effect. It states that the faster you discharge a battery the less capacity you have. So you start with a battery advertised as 130ah, but at 75 amps your capacity is 75 Ah. Another thing to keep in mind is cycle life vs depth of discharge. If you regularly pull 75 Ah out of the batteries they won't last very long. However if you only discharge to 50%, then you will get far longer life. So now your useable capacity is around 35-40Ah for a 135Ah pack. :(

4. C stands for capacity. A 1C discharge is the amps a battery deliver for 1 hour. C/20 is the 130Ah rating or in this case, 6.5 amps for 20 hours. A 2C discharge would be the rate of current the battery could deliver for 30 minutes, probably around 25-30 amps. It also works for charging just the same. C/8 would be the rate to charge the battery in 8 hours.

Hope that helps, this is only just scratching the surface of batteries but should get you started.

Greg
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191

NeilK
Posts: 10
Joined: Sat Oct 01, 2011 10:28 am
Location: Orkney

Re: Newbie battery questions

Postby NeilK » Thu Oct 13, 2011 6:27 am

Greg,

Thanks for the posting. VERY helpful. I do appreciate your time on this. :)

The car does have a Paktrakr so I will have a look at what that is saying over the weekend.

Also I will be immersing myself in the joys of the Peukert effect for bed time reading.

Any other contributions will also be gratefully received.

Neil


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