Quote:
Originally Posted by eq1
Is internal resistance some kind of tell-all metric for the quality of batteries/sticks?
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Sort of.
It's really hard to make a cell with a low internal resistance.
For our purposes, there are two reasons why a low internal resistance is desirable.
The first is simple. If you push current through a resistance the power has to go somewhere. Where does it go? It converts to heat. If that heat is greater than the internal structure of the cell can handle, parts of it will melt internally and cause shorts or breaks in continuity. The more resistance you have, the more heat you generate, or the less power you can successfully pull out of the battery.
The second is based on ohm's law V=iR. Voltage = amperage * resistance.
If given a chance, the cells will provide much more than 100A - until they burn out (see #1 above). They are limited to a max of 100A or so by the car. Now the car will draw up to a set amount of power (for example 100A) and watch the voltage of the cells so that it can stop before the cells are drained too much (protecting them from damage). There is a hard upper limit of 1.4V per cell (the limit of the chemistry) and a computer determined lower limit of 0.9V per cell.
What we see then is that under discharge the amperage will rise up to the maximum that the car allows and the voltage will plunge with the load. The car will lower the amperage draw as necessary to keep the voltage from dropping too low.
If the internal resistance is lower, the voltage is higher for a given amperage and the car takes advantage of it (or rather, doesn't have to correct for it). At the same time, less power is converting to heat. This is why a BetterBattery can hold 20 bars of discharge for so long - the voltage stays high enough for the car's computer as it maintains that amperage level, and it doesn't start to overheat.
Many cells that we tested and older, high resistance stock cells will have the voltage drop below the minimum at extremely low amperages. In the car, the BCM sees this and triggers a P1447-74 (2000-2004) or P1446-74 (2005-2006) voltage deviation error.
If the cells are worn out or damaged from heat (have vented), they will have a reduced capacity because major chunks of the internal surface area of the cell are no longer in-circuit (from melting) or have no electrolyte (from venting). In essence, parts of the cells are non-functional and do not contribute to the electrical load. (This is the outcome of reversing a cell - driving its' voltage below 0V). These result in P1449-78 (2000-2004) or P0A7F (2005-2006) (P1433 for a 2003-2005 Civic, P0A7F for a 2006-2011 Civic), because the cells don't meet the minimum standards for the car - they still work, but not very well.
When we tested KingKong's initial offerings, their voltage bottomed out instantly at 10 or more amps. I'm not sure what error the car would have thrown if we put a pack of them in, but it would have been instant and it wouldn't have been pretty. To be fair to them, most batteries perform the same way - Tenergy, Sanyo Twicell and about 40 other brands. The BetterBattery cells initially did poorly (40A), but they showed promise, so we worked with the manufacturer and they improved them over a two year period. They are now rated for 150A discharge, and can handle 120A sustained. The original cells are rated for 100A and can handle 90A sustained.
Now the reverse is true for charging. A high resistance cell will have a higher voltage AND convert more power to heat. That means that less amperage is consumed (pushed into the cell) over a given time. The BetterBattery, Bumblebee and Mile Hybrid batteries are lower resistance and therefore take more of the amperage and convert a greater percentage of it to chemistry (to potential charge).
What makes one battery have a better or worse internal resistance? The chemicals and metals used in construction make a difference, but the number one factor is the construction. More solid connections make for lower resistance. The problem is that you may be talking hundreds of tiny welds inside each cell to do a good job, which is not something that most smaller companies can accomplish. It also raises the price of the cells because of the labor and robotic equipment required. That's why a price below $12-$15 per cell should set off alarm bells. The price is low because they cut corners somewhere.
Does that answer your question?
