Re: Redox batteries

Jean-Marc Delaplace wrote:
Evgenij Barsukov wrote:
Jean-Marc Delaplace wrote:

You are right about the concern with the leaks. However, I still do not know how toxic vanadium is for the environment. Of course, containment measures should be taken in the design so as to avoid any spillage.

I want to answer your question about not using lead acid batteries. I hope this will raise an interesting debate.

First, let me recall that my purpose is to build a hybrid propulsion house boat. That is, a boat that will sometimes ride at top speed (15 km/h), sometimes at half speed (most canals are limited to 6 km/h) and sometimes it will stay at a mooring for days.

The objective is to be energy-efficient. The energy source will be one or more of the following:
-a diesel generator: 20 kW;
-photovoltaic cells: at most 1 kW peak;
-wind turbine: at most 500 W peak.

To achieve optimum fuel efficiency, the generator must be used near full power exclusively. Then, the role of the battery is to buffer this pulsed power source to deliver just as much power as needed when needed.

To save energy, using the generator should, as much as possible, be avoided and power should come preferably from the sun and/or the wind.

This is where the lead-acid battery has two major drawbacks:

1) A lead-acid battery should always be fully charged, i.e. it should be partially discharged for the shortest period of time possible, to avoid the degradation process to come to play. This is not compatible with waiting until sun or wind are available.


This is true. However, once your boat is moored, what prevents you from
connecting a charger to it and charging the batteries to full?
Deep cycle batteries can tolerate even full discharge, as long as
they are recharged soon to prevent crystallization of PbSO4.

2) A lead acid battery must be charged using precise current rates according to the state of charge. This is incompatible with the idea of accumulating all and as much energy is available at a time, i.e. always using the generator at nominal power.


Lead Acid battery is one of the least sensitive to charging rates (probably only cylindrical NiCd beats it in this respect, but they
are not available in right sizes). If you take the Lind book on batteries, there are over 10 different charging methods described for lead-acide, which have little in common. Which basically means, it does not matter how to charge it as long as you do not cause
1) persistent extensive gasing
2) persistent undercharged condition (unsufficient voltage)


What do you mean with being little sensitive to charging rate? Apparently you can charge a LA battery at C rate until 80% SOC, but the remaining has to be charged at about C/10 ("finishing and equalizing"). This is where the benefit of hybrid propulsion is lost: whereas I could charge 80% at full power of my generator, that is 0.8 h, to charge the remaining 20% at C/10 would require running the generator at 1/10 its power for 2 hours !

You are applying this formula too formally. The reason for the waiting period during last 20% SOC has really nothing to do with SOC itself,
it has to do with waiting for diffusion to equalize concentrations
and therefore it only applies to the case of continuous charge.

In hybrid application most of the charging is done opportunistically, e.g. when primary energy generator has some extra energy not used for propulsion. So first of all charging would be way below 1C rate
(most likely ~C/10), second it would not be continuous, which eliminates
the need for any waiting period to reach 100% SOC. If you charge bursts
from 1C to C/100, that average to C/10 over time, you do not need to
do any waiting at the end, you will end up with 100% charged battery.

Same applies also to continuous charge at low rate.

Lead acid batteries have been used in hybrid applications (e.g. diesel submarines) for almost a century and are still being used.


I would then spoil the efficiency advantage i was planning stating that the generator need be used at full power only for the best efficiency. If you consider that friction consumes more than 10% of the total engine torque, when you run an engine at 10% load it consumes two times more oil than required, which mean half the expected engine efficiency. In addition, you come into problems such as cylinder glazing, etc.

Even if charging is not opportunistic but continuous, problem of efficiency can be easily taken care of by switching off the generator completely to give time for concentration equalization, and than turning it back ON periodically.

Speaking about 1C rate (either charge or discharge), vanadium flow battery would require gigantic electrodes and flow rate to come anywhere close to this rate. All present implementations that I have seen are working in C/10 to C/100 rates.

In short, I think the constraint of slow charging over 80% ruins the advantage of using a hybrid propulsion chain. This i why I think lead acid is just not adequate for hybrid propulsion.

As shown before, this constraint appears only on paper. Correct understanding of the reasons of this charging recommendation shows that
it does not apply to this application.

Regards,
Evgenij
.

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