Regarding a storage battery allowing "pass through charging" ie, simultaneous charging from a power source [dynamo or wall plug] and constant voltage output for smartphone etc charging]
I have used a Cinq5 with good result , other than it lasted only about 6 months of continuous use before it could not be charged by my SON dynamo anymore. So the life of the internal battery is not great
Storage batteries made by VOLTAIC systems specify "pass through " function, so it does simultaneous charge and discharge.
I have one of these VOLTAIC batteries [plus solar panel], and it defiantly functions as specified, but it will have a limited lifespan.
More research lead to the "Soshine E45 18650 LCD USB ", which is supposed to allow "pass through charging"
check out this link
https://www.cyclingabout.com/buffer-batteries-pass-through-charging-dynamo-hub-systems/ This device does not include any batteries, so you buy those separately
This takes two 18650 lithium batteries which are user replaceable .This is a huge plus as they are easily available, from expensive new ones to very cheap recycled ones
i have just ordered the SOSHINE E45 on ALIEXPRESS , and was NZ$20 with free postage
So i will be field testing this set-up in the future, both charging from SON dynamo, and solar panel...... probably the limitation will be the quality/longevity of the electronics oi the device but for the money it is worth trying
Of the few regular lithium ion storage batteries i have had, they have all charged from a solar panel , but only the smallest capacity one [? i think about 1600mAhrs] would charge from my SON dynamo,
After some test use of the Soshine ES4..... there is one big problem ..... the "on/off" button will not turn the unit on [ but once "on", it can be turned off]
To achieve turning it on , the unit needs a current coming into it, which can be wall charging / solar panel or hub dynamo.
When riding , it will turn on at about 9 to 10 km/hr and bypass charges my phone as it is supposed to .
I read a "reddit" post about the ES4, regarding the not turning on problem , and one contributor mentioned that battery removal and short circuiting between positive and negative terminals with a small length of wire, then after the batteries are replaced, allowed the unit to be turned on with the on/off button.
Unfortunately mine does not do this
However..... i have come across yet another storage battery, this is the NITECORE NB10000,[10000MaHr] and after some enquiry found that this is suppose to simultaneously charge and discharge.
In the NITECORE range , the 20000MaHr specifies that it will simultaneously charge and discharge , but this information is absent from the small print related to the 10000mahr version...... but the supplier assured me it did do this
The great thing about this battery is they are very light , with carbon fibre shell, and some degree of water resistance
So i bought the NB10000 , and yes..... it bypass charges my apple cellphone just fine.