3 ms·

Modern cars with keyless entry typically draw ~50mA@12V when "off", this power is used to power the receiver, keep ECU state for such things as learned fuel map

Modern cars with keyless entry typically draw ~50mA@12V when "off", this power is used to power the receiver, keep ECU state for such things as learned fuel maps, blink the alarm-on indicator, etc. 45Ah capacity gets you a bit over a month to drain down the battery, while 10Ah gets you a bit over a week. This isn't good, you'd have to keep it on a battery tender if you're not driving the car for just a week. I can also see how one would implement a low voltage cutoff to prevent running down the battery, but then, how in the world do you automatically re-engage the battery to allow starting the car? You've shut off the power to the whole car, so you no longer have a circuit to monitor. If you're allowing some amount of current to trickle out, you must, by definition, be allowing the voltage to sag, which can run the ECU or alarm out of spec, causing havoc. As the battery designer, your only input to make these decisions is the resistance of the car's electrical system. The only thing you control is your own internal resistance.

conjecTech 11y ago
Co-founder here: The capacity isn't set in stone. You might be right, if we need to increase it we can - or just offer a couple different options, but so far we haven't seen any issues and a number of our beta users are only using their cars once every few weeks. The capacity quoted on lead-acid is also a bit deceptive. There amount of current you can draw drops pretty heavily with state-of-charge. You might have trouble starting your car even with 50% of charge left. So if Ohm doesn't work for you, you'd likely be having trouble with a lead-acid as well. As far as the low-voltage cutout - it's definitely not trivial, but it's been done in other applications previously. I'd be a bit ashamed if you were able to come up with a solution of the cuff though, it took us a lot of work to get it working! It does involve some energy injection, but not enough to cause problems.
- oppositelock 11y ago
  I can speculate, though :) You can periodically pulse the circuit at a low voltage and figure out the resistance, and if the voltage is low enough, you won't power anything up. When the ignition goes into the ON position, you've got a huge drop in resistance as fuel pump, and other necessary electronics get enabled. A sudden drop in resistance is a good signal that you should turn the power back on. This will not work with some more recent electronic systems, but should work great for most mechanical ones. Pretty sure you haven't solved things like the interaction with a BMW battery monitoring computer, for example, and those cars will probably reject your battery with some crazy error after a while. It's good that you've got space for more reserve capacity, parasitic drag is getting higher as cars get more fancy. For example, radiator fans and turbo oil circulators can run for quite a while after the engine is off. Your super capacitor design is elegant - you can charge the supercap off the battery slowly, even at low state of charge where its voltage would sag too low for the starter relay if you were directly using the battery. I have a little car which I race and weight matters, so I've been using a ~12Ah LiFePO4 home-made battery for a while (A123 cells). This is a very durable little battery chemistry. No fancy super-caps, though, I just keep mine topped off and starting the car puts the battery cells close to their maximum discharge current. I wish you guys the best of luck. Lead acids are pointless with modern battery technology.

conjecTech 11y ago

Co-founder here: The capacity isn't set in stone. You might be right, if we need to increase it we can - or just offer a couple different options, but so far we haven't seen any issues and a number of our beta users are only using their cars once every few weeks. The capacity quoted on lead-acid is also a bit deceptive. There amount of current you can draw drops pretty heavily with state-of-charge. You might have trouble starting your car even with 50% of charge left. So if Ohm doesn't work for you, you'd likely be having trouble with a lead-acid as well. As far as the low-voltage cutout - it's definitely not trivial, but it's been done in other applications previously. I'd be a bit ashamed if you were able to come up with a solution of the cuff though, it took us a lot of work to get it working! It does involve some energy injection, but not enough to cause problems.

oppositelock 11y ago
I can speculate, though :) You can periodically pulse the circuit at a low voltage and figure out the resistance, and if the voltage is low enough, you won't power anything up. When the ignition goes into the ON position, you've got a huge drop in resistance as fuel pump, and other necessary electronics get enabled. A sudden drop in resistance is a good signal that you should turn the power back on. This will not work with some more recent electronic systems, but should work great for most mechanical ones. Pretty sure you haven't solved things like the interaction with a BMW battery monitoring computer, for example, and those cars will probably reject your battery with some crazy error after a while. It's good that you've got space for more reserve capacity, parasitic drag is getting higher as cars get more fancy. For example, radiator fans and turbo oil circulators can run for quite a while after the engine is off. Your super capacitor design is elegant - you can charge the supercap off the battery slowly, even at low state of charge where its voltage would sag too low for the starter relay if you were directly using the battery. I have a little car which I race and weight matters, so I've been using a ~12Ah LiFePO4 home-made battery for a while (A123 cells). This is a very durable little battery chemistry. No fancy super-caps, though, I just keep mine topped off and starting the car puts the battery cells close to their maximum discharge current. I wish you guys the best of luck. Lead acids are pointless with modern battery technology.