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MG MGA - Odyssey PC925T Battery

Does anyone have any experience with the Odyssey PC925T battery? My 6 volts are on there last leg after 2 years and to expensive. I like the idea of not having to modify the battery holder.
WMR Bill

Hi Bill,

not a PC925T, but I've got a Odyssey PC680 on my A. It cranks a 2.1 litre 5 bearing over no problem.

I got the idea from my brother who use to run his Porsche 911 on a PC680.

They are about half the size of a standard 6volt battery, and fit into the battery tray with no modifiation of the tray, but have stacks of cold cranking amps.

Kind regards,

Grant :-)
G Hudson

Works every time! I didnt even bother charge it after the winter. Left it right in the car in my cold garage.
Jeff Bennett

Are you abe to use the existing hold down bar?
WMR Bill

First time I've seen the J-J pattern. Apparently that refers to the top handle configuration. The price on that page is almost $100 higher than other places.

360 CCA is only about 60% of what a modern lead-acid battery can do, but still enough for an MG (especially if it can produce close to that current in very cold weather).

The important point here is the moderate 27AH storage capacity. If you leave the lights on when parked, or the generator goes dead, the battery will go flat in half the time of the original twin 6's. It has close to the same power density of a lead-acid battery, but it is just one small battery, so only half of the original storage capacity. Two of the original twin 6's in series made 12V-58AH. Look up the Reserve Capacity rating in minutes.

The AGM (Absorbed Glass Mat) battery is sensitive to overcharging with voltage in excess of 14.7 volts, so it may have a shortened life when used with a generator and vintage relay-type voltage regulator. These batteries are much happier when mated to an internally regulated alternator that will not produce excess charging voltage. I don't know what happens to it if you apply 15.5-16.0 volts continuously (but I wouldn't want to try it).
Barney Gaylord

Would not two of the 925 equal the reserve capicity of the two 6 volt batteries? I believe so. Barney is correct in stating that half the size makes half the reserve capacity. Using two 925s in parallel will not increase the cranking power but will increase the cranking time available. However, it is always problematic when using batteries in parallel to maintain equal charge. In fact, if one battery in a parallel set develops a problem that reduces its voltage, it will pull the other battery down to that voltage, which can result in early failure of both batteries.

Barney, I don't agree with your assessment of the possible problems with an AGM battery and the old style voltage regulators. If the voltage regulator is operating properly, charge voltage should be around 14.5 volts max. I do agree that if the VR fails and creats an over voltage, the AGM battery is in danger, Being completely sealed a rupture of the case would probably occur.

I recently ordered a 925 for my project and will let the members know of my experiences.
James Johanski

Installing two 12v batteries in parallel would double the CCA current capability and well as the Amp Hours (and reserve capacity) CCA is not a problem in MGA with a single battery.

Installing two batteries in parallel is somewhat risky without some rather sophisticated controls in between. As James mentions, one battery going weak can drag down the other one. With batteries in good condition charge equalizing is usually not a problem.

The risky bit is that batteries may occasionally develop an internal short. If that happens with a second battery in parallel, then the faulty battery looks like a dead short across the good battery, and the ensuing high discharge rate could result in catastrophic destruction of both batteries (especially the good one). These Odyssey batteries are capable of generating more than 2000 amps on a dead short (2250 amps for 5 seconds). The resulting overheating and internal gas pressure could explode the good battery. The faulty battery might also explode due to resistance heating of the internal short.

Having two batteries in the car will double the probability of this type of battery failure. One battery with this failure just leaves the car dead on the road. Two in parallel when one has this failure could result in an explosion or possibly a burn to ground situation.

This is no joke. Several years ago my daughter had her car go dead very suddenly on the road with no notice, obviously a bead battery. When I tried to jump it there was a HUGE spark and instantly hot jumper cables. Sure glad I made that last connection on the engine block away from the battery. That battery developed a solid internal short without notice when it had been previously working fine.

One cheap safeguard against this condition would be a big fuse (500 amps) installed in one of the interconnecting cables between the batteries. That fuse has to be large enough to carry highest possible engine cranking current, but not much larger.

Another solution is a cut-over switch to connect only one battery at a time to the main cables, and possibly some diodes between the batteries to allow charging of both but discharging of only one at a time. These schemes pretty much defeat the use of two batteries at once for cranking or reserve capacity.
Barney Gaylord

The AH rating specification is highly overrated. On your typical factory stock MGA, I suspect the running current to be something just less than 3 Amps average, driving the ignition and fuel pump. Lets see, the typical AH rate is at a 20hr discharge rate to a 10V cutoff voltage. For a 27AH battery, this is 1.35 Amps. Less than the draw rate for the MGA. I suspect that a running MGA would still run below 10 Volts but there is little battery reserve left after it reaches this 10 volt level so we won't count that. So what does that mean for us? It means the battery will not last as long as 9 hrs in our application as the rating might imply. What can we expect? Well my guess is that ther would be something like a 75 or 80% derating, so we might get 6.75 hrs of actual running time on a fully charged, early-in-life battery. Hard to tell without seeing the load/life curves of a particular battery.

The AH rating is not really important in my world.

The CCA, or cold cranking amps is the specification that matters to me. Can the battery provide the starter current necessary to turn over the engine when cold.

Running AGM batteries (in general) at a higher than designed voltage, produces gasses at a rate faster than the recombination valve can work causing outgassing. This outgassing will dry out the electrolyte and definitely shorten the life of the battery. Some storage-batteries are spec'd to never exceed 2.25V per cell or 13.5 volts.

Wiring parallel/series AGM battery packs is a common practice in Uninterruptible Power Supplies and in telecommunication systems. Many take no real effort to isolate a bad battery. Some are intelligent enough to determine a problem exists and signal an allert that there is a fault, but continue to run anyway. To counter the effect of individual batteries/cells becoming weak, a higher than usual voltage is periodically applied for a short period of time, maybe up to 15 volts. This is still lower than could be found in an automotive system.

This all applies to AGM storage-rated batteries. If the battery is designed for automotive use, then their chemistry may require/allow for some slightly different voltages. Parallel/series connection of this type system may, or may not be recommended.

Just my opinion coming from the UPS industry years ago. We wired 10-12V 32AH AGM batteries in series for a 120VDC system and paralleled 3 strings to provide more running time. Each string was individually circuit-breakered to limit the fault current if one were to occur. I do know that when a 1/4" chain is dropped across the terminals of a bank of 6 batteries in series, it will instantly melt the links and the batteries too!

Chuck Schaefer

AH rating is important if your generator goes bad at night with lights on and heater and wipers running to deal with snow (been there). 60-watt headlights alone can draw 10 amps, parking lights 2 amps, dash lamps 2-amps, ignition coil 3 amps running, fuel pump maybe 1 amp (depending on type of pump), heater 3 amps, wipers 4-amps.

If you're counting that adds up to 25 amps, which exceeds the 19 or 22 amp rating of the generator when it is working (which is exactly why the MGA originally had two batteries). If the generator goes south that 27AH AGM battery goes below 10 volts in about 45 minutes (when it's new and starting with full charge). If it's older issue it might be a half hour or less before you have no headlights.
Barney Gaylord

This thread was discussed between 24/09/2009 and 26/09/2009

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