I think Dan uses a meter too, but his might be a different one.
I do indeed use a meter for
testing purposes, but once I have established charging rates (in current and voltage) I leave it home to minimize power losses due to increased resistance as a function of length and at the (additional) connectors, which can become poor and subject to problems if they become wet by rain or dew.
I rewired a couple of my meters with heavier-duty low-loss shielded coaxial cable and new clamshell connectors and found they worked much better. While they measure voltage and current as before, the actual resistance losses at the device being charged are now less. I check my meters with a separate volt/ohm meter, but if you don't have a VOM, you can gang multiple meters and see which ones are more or less efficient by changing the order. The meters themselves seem to have low parasitic losses, but the same cannot be said for their cables, connectors or -- sometimes -- soldered connectors (which are often "cold" or "disturbed" joints by my observation.
In my testing, aside from the actual charger being used, cable length and quality and connector/soldered joint quality were the greatest determinants in power reaching a pass-through cache battery or device batteries. It pays to use quality cables with good connectors and keep them as short as possible. I prefer cables with gold-plated pins. Yes, they avoid surface corrosion, but more generally they indicate higher quality materials are used elsewhere in the cable/device.
There is a good general caution to avoid charging in the rain when the connectors are wet no matter how or whether the pins are plated. Not only can water interfere with a good connection, it can encourage electrolysis at the connection when voltage is passed through, causing the pins in the charger to erode and eventually fail/become open. I have made a little "shower cap" for my Tout Terrain charger that addresses this problem, but other chargers address it by design. I put my cache battery and device(s) being charges inside the handlebar bag during charging for this reason. It is the easiest way for me to shield the connectors from water exposure and keep them secure on rough roads.
While I have a lot of experience with the Tout Terrain/Cinq5 "Plug" chargers, I am also testing another model that looks very promising at a low price. I will make a full report as soon as time permits me to complete my tests.
All the best,
Dan.