PD or Iota for my upgrade?
I'm thinking about a converter and battery upgrade. The batteries will be wet cell lead acid 6 volt Possible T-105s or the 230AH, EGC2 from Sam's club. I'll probably go for four. (Does nayone have...
Forum Discussion
There are so many good comments from knowledgeable people here, I scarcely know where to begin.
As to voltage -
The value reported in the stream is 14.5 volts unchanged for the last 45 minutes to hour - right after it starts to taper. For the hour before that it's 14.4. Yes, the voltage resolution is terrible. Even worse - the TriMetric will let you calibrate the voltage if it's off, and I haven't done that. It could be off and I recall thinking it might be off. I recall the voltage being 0.05 volts low off spec when I measured it with a good meter and thought spec was 14.6 when I wrote the post. Perhaps true voltage was 14.35, not 14.55. I'll check and calibrate for the next run. Even with poor resolution, we can interpolate from the data change points and periodic independent calibrations with a better meter. I will try to do that, but it may be a while. This is in my "Fun things to do" list - not the "Things that need to get done before the vacation trip" list.
I did emphasize this was preliminary :) but the comments are helping me fine tune the changes needed for the next run.
As to constant current and tapering -
The PD series has two limits - voltage and current. The PD circuit diagram of the PD9260 and 9280 are identical except for the resistor bridge that sets the current limit. The voltage limit circuits are identical. The components differ only in the max current carrying limitations. If the resistors limiting current to 80 amps in the PD9280 were replaced with the PD9260 resistors to limit current to 60 amps, I would have expected to have seen flat current limiting action at 60 amps for the first 3 hours, then drop off. With the PD9280 it was above 60 amps for that time.
I suspect we are seeing the PD 9280 in voltage limit conditions, not current limit conditions for the entire time, except for a few minutes at the very beginning.
Why didn't it get to 80 amps? I suspect we will find the answer is the low 14.4 (perhaps as low as 14.35 actual) Boost voltage limit in the PD coupled with the resistance of my 1/0 cables and the battery resistance itself. The PD will always increase voltage towards its voltage limit (Boost, Normal or Float) unless it first reaches its current limit. With the low voltage limit of 14.35, it hit that limit before the 80 amp limit except in the first few minutes.
I have several options to test this:
If the output voltage at the converter was steady at the voltage limit, we'll know it was in voltage limited state, not current limited state. That's easy to test - remember I didn't have a monitor on the converter voltage, only at the battery voltage end (although the TriMetric can be connected to monitor a second voltage, and I may run a monitor line to the converter so the streaming data includes that voltage - still at low resolution)
If that's what's going on, then reducing cable resistance should increase output current - that's always true as long as the converter is not in current limited mode.
I can also tweak the PD voltage limit a bit higher to its spec, since I recall it's 0.05 volts low, or I can be more daring and tweak it up towards 14.6 or even 14.8. I didn't recall there was that much difference between the Trojan recommendations and the PD. As soon as the PD voltage limit is set higher, it will increase its voltage, which will increase current until the current limit is reached at 80 A (or the new voltage limit is reached).
Changing cables is actually harder for me than tweaking voltage settings, but DW will kill me if I blow up the PD just before we leave. :)
Thanks again for all the comments. I would dearly love to run another test this weekend with some better calibration/control. We'll see if time permits.
As to voltage -
The value reported in the stream is 14.5 volts unchanged for the last 45 minutes to hour - right after it starts to taper. For the hour before that it's 14.4. Yes, the voltage resolution is terrible. Even worse - the TriMetric will let you calibrate the voltage if it's off, and I haven't done that. It could be off and I recall thinking it might be off. I recall the voltage being 0.05 volts low off spec when I measured it with a good meter and thought spec was 14.6 when I wrote the post. Perhaps true voltage was 14.35, not 14.55. I'll check and calibrate for the next run. Even with poor resolution, we can interpolate from the data change points and periodic independent calibrations with a better meter. I will try to do that, but it may be a while. This is in my "Fun things to do" list - not the "Things that need to get done before the vacation trip" list.
I did emphasize this was preliminary :) but the comments are helping me fine tune the changes needed for the next run.
As to constant current and tapering -
The PD series has two limits - voltage and current. The PD circuit diagram of the PD9260 and 9280 are identical except for the resistor bridge that sets the current limit. The voltage limit circuits are identical. The components differ only in the max current carrying limitations. If the resistors limiting current to 80 amps in the PD9280 were replaced with the PD9260 resistors to limit current to 60 amps, I would have expected to have seen flat current limiting action at 60 amps for the first 3 hours, then drop off. With the PD9280 it was above 60 amps for that time.
I suspect we are seeing the PD 9280 in voltage limit conditions, not current limit conditions for the entire time, except for a few minutes at the very beginning.
Why didn't it get to 80 amps? I suspect we will find the answer is the low 14.4 (perhaps as low as 14.35 actual) Boost voltage limit in the PD coupled with the resistance of my 1/0 cables and the battery resistance itself. The PD will always increase voltage towards its voltage limit (Boost, Normal or Float) unless it first reaches its current limit. With the low voltage limit of 14.35, it hit that limit before the 80 amp limit except in the first few minutes.
I have several options to test this:
If the output voltage at the converter was steady at the voltage limit, we'll know it was in voltage limited state, not current limited state. That's easy to test - remember I didn't have a monitor on the converter voltage, only at the battery voltage end (although the TriMetric can be connected to monitor a second voltage, and I may run a monitor line to the converter so the streaming data includes that voltage - still at low resolution)
If that's what's going on, then reducing cable resistance should increase output current - that's always true as long as the converter is not in current limited mode.
I can also tweak the PD voltage limit a bit higher to its spec, since I recall it's 0.05 volts low, or I can be more daring and tweak it up towards 14.6 or even 14.8. I didn't recall there was that much difference between the Trojan recommendations and the PD. As soon as the PD voltage limit is set higher, it will increase its voltage, which will increase current until the current limit is reached at 80 A (or the new voltage limit is reached).
Changing cables is actually harder for me than tweaking voltage settings, but DW will kill me if I blow up the PD just before we leave. :)
Thanks again for all the comments. I would dearly love to run another test this weekend with some better calibration/control. We'll see if time permits.
