VRLA / SLA charging under freezing temperature

Up to about State of Charge (SOC) = 80%, the charging current for a Lead Acid battery can be set to say 0.1..0.2*C. So you know that within a certain time the battery accepts a certain charge.

The last 20% takes longer as the current is determined by the battery itself to avoid gassing. I know that the time to reach say 95% can be reduced somewhat by applying the "cycle" charge voltage instead of the "floating" voltage. I am also aware of the negative temperature coefficient of around -3 mV/cell/celcius.

Question:
When the battery is at say -15 degrees centigrade (5 F) and I apply the negative temperature coefficient (so about 14.2V for a 12 V battery), will it remain in bulk charge phase up to around SOC = 80%? In other words will it still accept 0.1*C up to SOC=80% without exceeding the "cycle" or "float" voltage limit?

Reason for the question is a small PV system. We have sufficient power, but during winter the charging time is limited to around 5.5 hours (daylight). When the battery will not accept say 0.1*C, or 0.05*C up to about SOC=80%, we can't get the energy into the batteries. If this is true, we need a larger battery bank to store the energy. The batteries will not be heated to room temperature, so they may cool down to -13 degrees Centigrade.

Both ESR and capacity degrade at COLD TEMPS and at -15'C Capacity is reduced to 60% of room temp. and near 0% at - 50'C . So I would add storage capacity if there is sufficient charge capacity in winter.

Data on reduction of capacity and increase of ESR is available. The design will account for these. However the increase in charging time to say SOC=80% at very low temperature is a mistery. For this application increased ESR during discharge is not a major problem. We will design for SOC>50% under worst case condition.

The problem may be in the variation of PV panel output. On the average it will be fine, even during November-January. However we may have periods of cloudy weather with snow so power output may be very low for more then a week. When the weather changes, the energy will be available during short periods of time (also because of horizon obstruction). It would be sad when we have the power, but the batteries will not accept it due to low temperature.

We are considering installing large amount of storage, but from an environmental and logistics standpoint, this is not attractive. So if you have some data or link on charging time increase under below zero (32 F) temperatures, it would be helpful.

Do you have temperature sensing? You'd think that an aggressive
charge current couldn't help but raise cell temps (in your favor).
Perhaps some sort of thermostatic control would be useful, even
(say) scavenging the panel backside heat and forcing it down a
duct to the battery case when the temperatures make it useful
(maybe, too, the opposite in summer)?

Temperature sensing is not planned now (but can be done).

Regarding the aggressive current, in fact that is the question. Will there run sufficient charge current without exceeding the gassing voltage?

I forgot to mention that the charger will have temperature sensing for handling the about -3 mV/cell/degr.C ....

Insulating or keeping the batteries warm, ought to be considered.

Something to be mindful of...
A discharged battery has a weaker acid concentration. It will freeze at a higher temperature than a charged battery. For this reason batteries ought to be kept fully charged in sub-freezing temps.

Unfortunately the tendency will be to deplete batteries in winter when you need power and it is more difficult to keep them charged.

I have not seen a table telling what freezing temperature equates to what charge condition. The general caution is that a fully discharged battery is practically water, hence it freezes at 32 deg F.

Check the diagram on this site> http://www.sulphuric-acid.com/techmanual/Properties/properties_acid_freezingpt.htm and calculate the Sulfuric Concentrations of Charged and discharged SLA/VRLA to figure out should you use the battery in certain temperature ranges.

The batteries will not experience below -15 degr. C and the design is such that under very worst case situation SOC > 50% (to avoid freezing and sulphation).

The problem is in in the charge acceptance at low temperature. I can install whatever amount of PV power, if the batteries won't accept the charge at say 14.3 V (below gassing at -15 degr. C) large PV panels will not help me. If charge acceptance is a problem, I need more batteries in parallel (or a larger single battery).

Manufacturers show nice charging curves at around 20 degr. C that show that up to say 80..85% SOC you can charge with certain current. However they say nothing for freezing temperatures. Only some makers of pure lead-tin batteries show information that indirectly shows they accept charge well at low temperature, but these are expensive.

@SunnySkyguy: There will be some thermal insulation, but this will not bring me 10 degr. C temperature rise.