Extension battery 2x Diodes or 2x 2way switches?
Postby ProDigit » Fri May 25, 2012 2:55 pm
Hi all,
I'm thinking of extending my e-bike's range by buying an extension battery.
There are 2 possible solutions to connect the extension battery. Via diodes or via a switch.
Via diodes I can connect both batteries in parallel. But the voltage drop of batteries lowers the more batteries are connected in parallel, meaning the controller won't shut down, until the batteries are really really empty (which is bad for battery life); and there's no way of knowing when the batteries are empty, until they are (since the battery level indicator will only drop when both batteries are already below their lowest charge).
With a switch it's somewhat safer. have 2 switches installed. Drive on the first battery, until it gives in. Switch to off. Switch the second battery on, drive to stop.
Now you know that both batteries are empty, and if you're still not at your destination, then there's no alternative than to switch both battery switches on, and drive with batteries in parallel.
Advantages of this setting is: no voltage drop of diodes.
Disadvantages: if the controller shuts down in parallel mode, the voltage per battery pack will be even lower than with diodes. One could literally drive both batteries dead, especially when one forgets to disengage the backup battery pack!
1- Diode: but the guy making the diodes, specifically made for bikes, does not make them anymore; So I thought of going on amazon to find one, and I found these:
http://www.amazon.com/D92-02-Integrated ... 81&sr=1-55
Instead of buying a 10-pack of 6A diodes that I'll have to solder together in parallel, I just thought of getting two of these (one for each battery's anode).
The only issue with these is that they look like a transistor!
They have 3 pins instead of 2. I don't know which pins to choose, as a diode usually does not have a gate. I suspect one of the pins is a grounding pin?
2- Switches:
I haven't yet found tiny switches that are easy to install and can handle 20A, if not I'll have to go with regular lightbulb switches,and hope they can handle the current.
3- Connection:
I'll also have to figure out a way to plug and unplug the extension battery from the bike for charging purposes.
Why would the controller shut down? Diodes tend to waste .7 volt to maybe a volt and a half at what ever current. I had an EMBI that had a switch for battery bank A and switched to battery box B. I thought that was dumb so I paralleled batteries and got doubled range. After that I found Hawkers and got more range. Help me to figure out why controller would shut down? I found welding diodes on eBay for under ten bucks. 100 AMP at 600 volts. Batteries tend to operate from a stand point of Peukert exponet. A 20 Amp hour battery gives you an amp for 20 hours but not 20 amps for one hour. Maybe 30 minutes. But not an hour. It might give you 40 hours at 1/2 amp but not a lot in a short time.
I couldn't afford to buy the 38 AH batteries that died in my xm-3000 and used 20 AH batteries. I got close to half range. Again close enough. I squished another set in parallel and got back my original range. Maybe a tad bit more. i don't need the range now but need the room for groceries so i am going to go back to 20 AH pack plus it charges faster and I now will have room for groeries.
maybe I missed what you are saying or impying.
KB1UKU
Controller shuts down when battery voltage drops between a specific point, in order to protect the batteries from being undercharged.
Because of diodes, there is a 1,4V battery drop, or less when only putting a diode on one terminal.
Another issue I'm struggling with, is deciding if my charger will be able to fully charge both batteries when in parallel.
If not, I'll have to disconnect them to charge them.
Lets try charging scenerio first. If it takes 4 hours to charge, maybe two batteries might require 8 hours to charge. if you need less time then increase charger size to 10 to 15 amperes. Now for diode diodes. Well I can only see using what little electrons there are and not wasting any as heat and voltage drop. Now for switch. Again, I see better Peukert exponet using two baattery pack vs carry extra dead weight and kicking in "new" battery pack. How many volt pack? What is Watts Wattage? Of controller? Battery pack Amp Hour? your normal charge time and charger current ? output of course. PPPPPP
KB1UKU
Problem is some chargers are rated at a specific amp rating.
I think I've read when the stock charger does 40Ah,and when putting 20Ah battery in parallel with the stock 40Ah battery, that the charger does not charge much beyond 40Ah (but trickle charges it).
So in other words, if 80% of the stock battery gets charged by 4 hours, and it takes another 4 hours to trickle charge the stock battery,
In this case (the numbers I'm about to use are just for approximation, not to be taken as exact values), charging an extra battery with the stock battery should take 6Hours upto ~80%, after that, trickle charging can take much more than another 6 hours!
Meaning, if you charge the battery only 8 hours a day, you might end up never completely filling the battery.
I do not believe the timing goes linear, meaning if the stock battery (40Ah) fully charges in 8 hours, automatically the same charger would charge a 60Ah battery in 12 hours, I do not think that is true. Rather it might fully charge in 15 to 18 hours, or perhaps even never fully charge the battery, as the battery capacity might be too large for the charger.
But my biggest worry of them all is, I don't know how the charger would respond in following situation:
Imagine after 8 hours of charging, that the stock battery would be 100% charged, and give me 30 miles.
If this would not be enough for me, I'd have to plug in an extension battery.
With the extra battery, imagine after 8 hours of charging, the bike still goes 30 miles, as the battery is only 80% charged, and not 100%, and thus, that buying the extension battery is in vain...
Just to complicate things more, dont forget the loss of efficiency in range due to the extra weight of the additional pack, decreased handling due to the extra weight, and storage space, as already pointed out. If making a bunch of short trips as opposed to one long one, then a second battery being swapped out, ride for charge might help some, but would be pain to swap all the time.
Adding an extra battery in parallel with the stock one, allows you to have more amps available, resulting in greater torque, which is better for acceleration.
The losses of carrying an extra battery are minimal, as the heaviest 20Ah battery weighs just 10kg, the 10A weighs less than 5kg. So it's like carrying an extra helmet with you, the bike generally suffers most from doing a lot of start-stops, which is what the extra battery will more than compensate for.
Even if it's range might be limited, at least more peak amps are available!
On good tires, and a flat road, having more batteries is always better, even if they are Lead acid.