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Originally Posted by redpoint5
Apologies for the thread-jack, but these cells look very interesting and I'm considering using them to replace my conventional group 51R battery in the TSX. Would these cells be ideal for an alternator kill switch setup?
If I assume a 5% increase in FE using an alternator kill switch, I would save approximately $5/month in fuel. This would be a long payback when considering the batteries, BMS, charger, etc. Still, it would be fun to reduce a little weight and play with the technology.
My conventional battery has an 85min reserve, which I haven't figured out how to convert to A/hr rating yet. I'd need to size my battery to run my car for up to 1.5 hrs at night. Thoughts?
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You could certainly replace your lead acid battery with these cells. I don't think you'll ever come out ahead financially though, if you do you'll at least be on your next vehicle by then. Thankfully lithium batteries last longer than lead acid, so this might actually be possible.
I wouldn't go at all by the reserve capacity on your current battery which is just a starting battery and not designed at all to be deep cycled. What you really need to figure out is what your current draw is going to be so you can size your battery accordingly. With the TSX being a bit more of a luxury vehicle, I'd assume it gobbles up more electricity than the average econobox. The best thing to do would be to get a clamp ammeter (or other ammeter) and see how much power your car draws while running with the headlights on.
As an example, I'm going to guess and say that it draws 40A. So, in one hour you use 40Ah (40 amps per hour). In 1.5 hrs you use 60Ah. These cells are rated at 20Ah, and as you can see, at least these ones aren't quite 20Ah. Also, you never discharge a battery completely. This extends the battery's life by quite a bit. For lithium you can use 70-80% of its capacity. 70% will give you longer life, and some additional buffer in case you need more than 1.5 hrs, so lets use that. Now, we have ~18Ah batteries (per my testing), and we can only use 70% of that. So, we actually have 12.6Ah of usable capacity. As we calculated before you need 60Ah of capacity. So, we can wire the cells in parallel to add capacity. To get the minimum required 60Ah we would need to parallel 5 cells to get 63Ah of usable capacity. So, you have 5 cells in parallel, and 4 cells in series for a total of 20 cells @ $27.50 (the price I paid) each. That comes out to a $550 battery (~10 year ROI based on $5/mo savings). If you go with the 80% depth of discharge (vs 70%) you could get 57.6Ah and only parallel 4 batteries and reduce your cost by ~$110. If your car only pulls 30A it would drastically change these numbers, so you need to know how much power the car pulls.
Alternatively you could buy a group 31 lead acid battery for about $100 and have close to the same capacity. Most group 31 batteries have just over 100Ah (~105-115Ah). But, with lead acid batteries you shouldn't take them down below 50% depth of discharge to get good life, so you would have around 55Ah of usable capacity. However, you'd need to replace the lead acid more often and it would of course weigh more.
IMO its probably not worth it ($ wise) for an alternator delete unless you take into consideration using it in your next vehicle. Its definitely not worth it for a starting battery because all you're saving is weight. I did this to start messing around with these batteries, figure out an assembly technique, and get comfortable working with the cells so I can use them on a larger project.